In-situ stress measurement
Motivation:The in situ stress measurement during deposition, ion implantation and annealing of thin films is of great importance because:
- (compressive) stress is the main reason for delamination, cracking and failure of thin film based structures
- the mechanisms of stress build up during deposition of nano crystalline and amorphous films as well as the correlation between deposition parameters and resulting stress are not well understood
- the stress in thin films is not necessary constant across the whole thickness, therefor ex situ measurement cannot deliver the interesting information
- to study the influence of deposition parameters and post deposition treatments on the stress dynamical in situ methods are necessary, especially for processes with bad reproducibility
Data analysis:The stress s in a homogeneous thin film is usually calculated by Stoney´s formula:
where Ys is the biaxial elastic modulus of the substrate and df, ds is the film and substrate thickness, respectively. Eq. 2 is an approximation for the case that df « ds and that the width to length ratio of the cantilever is small. S is the averaged (global) stress. The global stress S is related to the instantaneous stress s (z) by:
The instantaneous intrinsic stress s (z) is calculated using Eq.(1) and (2) from:
Examples:The in situ stress measurement technique can be utilized in all common deposition environments due to the fact that the measurement is not sensitive against high electric fields or temperature changes. Actual the instrument is mounted on a IBAD chamber which is connected to a high current implanter (DANFYSIK), so the stress evolution during film deposition and implantation can be studied.
To test the setup the build up of stress in Silicon during Argon ion implantation (EAr+=153 keV) has been investigated (see Fig. 2). The maximum compressive stress of 250 MPa has been build up after a fluence of 1.9x1014 ions/cm2 and is in agreement with data reported in the literature. The damage in the implanted region causes the build up of compressive stress. During further implantation plastic flow leads to stress relaxation.
In Fig. 3 the evolution of stress in a boron nitride (BN) film is shown.
Together with TEM analysis the growth of t-BN (turbostratic) and c-BN (cubic)
can be distinguished. TEM micrographs reveal a t-BN/t-BN+c-BN/c-BN layer