Effective method for stress reduction in thick porous silicon films

Abstract

A promising stress control process is demonstrated to achieve near-zero stress levels in thick porous silicon (PS) films. Stress reduction is necessary for thick PS structures to be used for radio-frequency applications such as on-chip inductors and transmission lines that require very thick (>100 μm) insulating materials. This study employs a standard sample structure with 50-μm-thick PS films formed on p+ substrates through anodization under 50 mA/cm2 and 25% HF concentration. Mass spectroscopy analysis shows that the desorption of hydrogen is correlated with the stress evolution in PS films. As-prepared PS films are under compressive stress. Thermal annealing in N2 ambient turns it into tensile stress. Further annealing in an oxidizing ambient restores the compressive stress. Such stress evolution can be explained by the atomic structure changes on the pore walls of PS films: H2-passivated and oxidized Si surfaces result in compression in PS films, whereas reconstructed state results in tension. These findings are used to fabricate thick PS films with negligible stress (<1 MPa) at room temperature.

Stress control of porous silicon films for microelectromechanical systems

“…The data shows two peaks in the desorption of hydrogen with increase of annealing temperature, starting at ~350 ˚C. However the data in Figure 5 have measured an increase in tensile stress [6,7], supporting our observation of the role that N 2 plays in this region. In region C, significant dihydride desorption occurs from the pore walls, so that upon removal from the thermal annealer to undertake measurements (as was done here), oxidation occurs which increases the compressive stress.…”

“…After annealing, the PS Bragg peak shifted away from its original position. Moreover, the peak for the sample annealed at 300 ˚C moved to the right of the Si peak, indicating the film became tensile as predicted by previous results [6]. However, the peak moved back to within 3×10 -3 degrees of the original peak after the HF dip.…”

“…Previous PS film stress studies focused on either in situ ultrahigh vacuum (UHV) [5] or treatment in N 2 below 350 ˚C [6,7], and thus did not investigate how PS stress evolves at temperatures higher than 475 ˚C [8] due to continued hydrogen desorption without ultrahigh vacuum conditions. Though the temperature dependence of PS stress and its relationship with hydrogen desorption have been reported [9,10], changes in stress after relatively high temperature annealing in N 2 or after repeated HF exposure, as would be common in a multistep PS-MEMS fabrication process, have not been investigated.…”

Thermal Properties of Porous Silicon

“…However, it should be noted that most of the studies have concentrated on changes of the chemical state of internal surfaces and the related strain and stress evolution in PSi films during thermal annealing, without suggesting a solution to control the stress. The real deal how to control the stress in PSi films was made only by Kim et al (2002). The approach proposed by Kim et al (2002) consisted of multithermal annealing and oxidation cycles.…”

Porous silicon for electrical isolation in radio frequency devices: A review

“…38 Kim et al reported an effective annealing process to reduce significantly stress in thick pSi films. 39 But, this technique requires a precise control of the temperature and annealing ambient (N 2 and O 2 ).…”