Combined Al-protection and HF-vapour release process for ultrathin cantilevers

Cantilever-based detection of ultrasmall forces is of great interest for several applications such as magnetic resonance force miscroscopy (MRFM) where an ultrahigh sensitive cantilever is used to detect magnetic resonance in small ensembles of electron or nuclear spins. Detecting the smallest possible forces requires ultrahigh sensitive cantilevers with low spring constant (k < 0.1 mN/m), resonance frequency f > 1 kHz and high quality factor (Q > 30000 in vacuum). Considerable progress has been made in fabricating attonewton sensitive cantilever but the yield of batch fabrication remains limited due the fragility of the cantilever during the fabrication. A new process fabrication of single-crystal silicon (Si) cantilevers is presented here that lead to a yield superior to 70%. It is based on a combination of two novel process steps for ultra-thin MEMS/NEMS fabrication, i.e. i) protective embedding of the structural Si device in aluminium (Al) during critical process steps and ii) HF vapour release to avoid the stiction effect. SOI wafers with single-crystal silicon thickness of 340 and 500 nm were used. Cantilevers were patterned with photolithography and dry etch process. 2-um-thick Al films were deposited on the front and backside of the wafers. The Al backside layer, patterned with photolithography and Al wet etching, acts as a mask for the deep backside etching while the Al top layer protects the Si cantilever and reinforces the SiO2 etch stop layer. Deep dry etching of the wafer backside is performed in a high inductive coupled plasma reactor. After removing the Al by wet etching the thin membrane of SiO2 is etched with HF vapour, a convenient technique to release fragile, suspended structures and which does not require fluidic water cleaning. This allows increasing the yield to more than 70%. A laser beam deflection setup implemented in a vacuum chamber is used to measure f and Q. The ring down method is used to calculate the Q factor. The cantilever is excited at its natural resonance frequency with a piezoelectric actuator with a precise pulse duration. After the excitation is interrupted, the decay time is measured and the factor Q is deduced. Several measurements have been carried out in different conditions: air, vacuum, with and without annealing. The characterized cantilevers show promising results in vacuum at room temperature, e.g. a cantilever (L=500um, w=10um, t=0.5um), respectively has f = 2kHz, k = 0.0004 N/m and Q > 100000. The force sensitivity has been calculated from the expression given by F~ (kT/Qf)^1/2 and is 6x10-17 N/Hz^1/2. The presented technology that combines embedding with Al thin films and the HF vapour releasing is a promising generic method for improving the fabrication yield of fragile freestanding structures. Annealing rearranges the atomic structures, reduces the dissipation energy and allows increasing the Q factor significantly.