A method to improve pattern critical dimension (CD) and reduce the metal residue level during gold (Au) and chrome (Cr) wet etch processes in MEMS devices is investigated. Instead of traditional single-step wet etch process, a multi-step wet etch method is used to etch Au with the etchant formulated by potassium iodide, iodine and deionized water (KI/I 2 /H 2 O). Compared with the single step wet etching process, the Au CD uniformity has been significantly improved from 10 um to 3.5 um in the experiment. Furthermore the Au undercut uniformity has been improved to 2 um which meets the requirements of mass production. Different etchant concentrations are also investigated to study the correlation between etch rate and potassium iodide concentration, and the efficiency of metal residue removal. In the chemical conditions of I 2 : KI: H 2 O=4:15:81, it is found that higher KI concentration brings AuI faster dissolution rate and metal residues are completely removed.
Poly-SiGe is widely used in MEMS applications as structural films for it's similar to poly-Si and compatible with standard CMOS (Al interconnects) backend processing. However, with its large as-deposited stress, it becomes more and more challenging and critical to control stress relaxation for sensitivity improvement of MEMS. In this paper, we demonstrate eliminating the stress in poly-SiGe films used in pressure sensor as membrane by excimer laser thermal annealing (LTA). First, the threshold melting laser energy density (ED th ) of poly-SiGe, the correlation between melting depth to laser ED and the thermal profile are study by simulation. Then the bare and real device wafers are processed by LTA (308 nm, 150 ns). It is demonstrated that stress can be released to the target exactly by setting a proper ED for the stress are increasing linearly with laser ED. And there's no contact open concern for surface temperature of AL is lower than 400C. Furthermore, the laser annealing seems to reduce surface roughness. Significantly, LTA is promising technique for surface properties modification in MEMS applications.
The effect of a macromolecular additive on the etching rate of aluminum (Al) horizontal etching in the wet process was investigated in this work. The horizontal etching in the Al wet etching process became more evident as the film Al becomes thicker. The proposed macromolecule additive, including polyethylene-polypropylene glycol, was added into the Al etchant solution to reduce the Al horizontal etching rate (ER). The undercut problem during metal patterning can then be improved. By using this method, the Al horizontal ER was reduced from 2.0 to 0.9 μm per minute and the selection ratio between the horizontal and vertical ER was effectively improved from 3 to 1.3 times. As well, a hypothesis of physical mechanism for the improvement was proposed. The dispersed particles from the additive emulsion inhibited the transport and exchange of liquid in a horizontal direction. This work provides an alternative reference to improve the selection ratio performance in the metal wet etching process compared with that when using traditional method.
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