Principal investigator: Dr Chen Longqing
Co-investigator: As/Prof. Miao Jianmin
Date started: 11/1998
Date completed: 11/1999
To develop low stress or stress-free polysilicon and oxide/polysilicon films for MEMS applications
Single Polysilicon Layer
In figure 1, the residual stress in as-deposited polysilicon film shows a significant dependence on the deposition temperature and pressure. The polysilicon films deposited at the lower temperature (560?C and 580?C) show compressive stress. As the deposition temperature is increased (605?C and 620?C) a transition region is revealed where the stress becomes tensile and then compressive. Figure 2 shows the influence of annealing on residual stress in polysilicon films deposited at different temperatures. For polysilicon film deposited at 560?C and 580?C, annealing at 650?C results in a remarkable change of the residual stress due to volume contract of polysilicon film when it transforms from amorphous to crystal during the annealing. As annealing temperature is increased, the residual stress in all polysilicon films is gradually decreased and it essentially is zero after annealing at 1050?C for 1 hour.
Fig. 1. Residual stresses in LPCVD polysilicon versus deposition temperature
Fig. 2. Influence of annealing temperature on residual stress in LPCVD polysilicon
Boron-Doped Single Polysilicon Layer
Figure 3 shows the residual stress change in the polysilicon films during the boron doping after pre-annealing. It can be seen: (1) there are not significant doping stress in the polysilicon films which are deposited or pre-annealed at higher temperature (more than 620 ?C for deposition and more than 1000 ?C for pre-annealing); (2) a larger change of the film stress can be obtained from the polysilicon films which are deposited and doped under the same process condition but are pre-annealed at different temperature. And it can be seen from figure 3, the lower the deposition temperature of the polysilicon films is, the larger the change of the film stress is. This is quite important for stress control in a multi-layer diaphragm structure which includes polysilicon film.
Fig. 3. Influence of annealing on residual stress in boron doping polysilicon films
Boron-Doped Polysilicon/Oxide Double Layers
Based on the above investigation results, a process sequence is presented which can well control the stress in multi-layer polysilicon structures. The property and magnitude of the residual stresses in highly boron doped polysilicon/oxide double layers can be controlled in certain range by use of the process sequence, as shown in figure 4.
Fig. 4. Residual stress in boron-doped polysilicon/oxide double-layer diaphragm structure versus annealing duration.