Design, Fabrication And Characterization Of The Ultra-sensitive Microcantilever

As a MEMS detector, microcantilever has been widely employed in force detection, mass detection, acceleration detection, pressure measurement, biosensing and so on. Typically, a ultra-sensitive microcantilver is proposed to detect the attonewton (10-18N) force, such as in the detection of single electron spin and nuclear spin based on megnetic resonance force microscopy (MRFM). The minimum detectable force of the microcantilever is limited by the thermo-mechanical noise. Based on this, a long, narrow and thin microcantilever with a high quality factor (Q factor) will be a perfect ultra-sensitive force detector at low temperature. However, the slight sizes will restrict the increase of the Q factor, which should be considered during the design of the microcantielver. The spring constant of the ultra-sensitve microcantileve is 1000 times smaller than that of the typical cantilever in atomic force microscopy (AFM). Thus, it is fragile and easily to break during the fabrication. Moreover, there is a lot of work to be consummated on the research about the quality of the ultra-sensitive microcantilever.This dissertation focuses on the ultra-sensitive silicon microcantilever to improve the yield of the fabrication, optimize the structure, and consummate the quality research. Accordingly, the design, fabrication and characterization of the ultra-sensitive silicon microcantilever have been widely investigated in this dissertation.The ultra-sensitive silicon cantilever has been designed based on the energy dissipation model and the thermo-mechanical noise theory. The vibrational energy can be bissipated through the airflow loss, the support loss, the thermoelastic loss and the surface loss. The loss in airflow is negligible in vaccum below 1 Pa, and the surface loss will dominate the Q factor of the microcantilever as the thickness decreasing. The surface loss is much larger than the other enengy loss for the investigated microcantilever here. Finally, the ultra-sensitive silicon microcantilever to detect attonewton force is designed based on the thermo-mechanical noise. The silicon microcantilever has been optimized for the application in magnetic resonance force microscopy (MRFM). The reflectivity of the microcantilever will be the best, when the thickness is an odd multiple of quarter-wavelength. The field gradient of the permanent magnetic tip is calculated by simulation based on Ansys. A conical magnet shows a stronger field gradient in the near field than spherical magnet and cylindrical magnet, and the field gradient in the near field will increase as the dimensions of the magnet decrease.An SOI-based fabrication of the ultra-sensitive silicon microcantilever through wet etching has been proposed. The silicon microcantilevers are protected by the surface silicon oxide and black wax. The surface oxidation can protect the silicon microcantilever, meanwhile, also can thin down the microcantilever to an appointed thickness. The buried oxide (BOX) layer is found to crack at the end of the bulk etching due to the large internal compressive stress formed during the thermal oxidation. It leads to the breaking of the Si cantilevers on the BOX. Hence, patterning the BOX has been proposed to release the internal stress to avoid unexpected cracking of the BOX film. In other words, the BOX film cracks at the appointed place to avoid the microcantilevers breaking. The BOX layer is patterned to blocks and cantilevers respectively. The yield of the microcantilevers is less than 50% using this block patterning method. For the cantilevers patterning, the patterned BOX cantilevers were 550μm long and 50μm wide, for the designed 500μm long and 10μm wide silicon microcantilevers in the experiments. Based on this method, the yield of the silicon microcantilevers is as high as 100%. The SOI wafer was putted in a PTFE container during etching and cleaning. We moved the container from liquid to liquid instead of moving the wafer, to protect the cantilevers from breaking. Finally, the microcantilevers are pulled out from the ethanol in the end and dried on a hotplate. The fabricated microcantilevers are 250-500μm long, 10μm wide and 0.5μm-1μm thick respectively.The stiffness, frequency, Q factor and mass-loading effect have been investigated in free air space. The stiffness is low, thus the microcantilever is sensitive to the surface stress. For the microcantilever of 465x10x0.85μm3, the deflection is found to be 14μm, when the deposited Au film is 48 nm thick. The energy loss in free air space is dominated by the air damping, and the Q factor of this microcantilever is as low as 7.25. We glue mass on the tip of the microcantilever to research the mass-loading effect. Compared with the mass formed by micromechanical machining, gluing mass can load various particles on the same microcantilever. The Q factor is enhanced from 7.25 to 19.07, when the mass is 22.82 ng. The mass loaded here dereases all the flexural vibration frequencies of the microcantilever, which is different from the bulk mass formed by the micromechanical machining referred in the references. The effect on stiffness should account for the difference. Typically, the resonant frequency and quality factor are 5.6 kHz and 6.3×104 respectively at 77 K, for the microcantilever of 465x10×0.85μm3 in vacuum chamber. The drift of the resonant frequency was 20 Hz in this temperature range (77-220K). The cantilever shows a low quality factor around 140 K, compared with 115K to the bulk silicon [22]. The suppression on Q factor due to mass-loading in vaccum has been found, which confirms that the surface loss is related to the vibration period. The microcantilever and the polished fiber end form a Fabry-Perot (FP) cavity. The detuning of the cavity can suppress the noise, thus cool down the microcantilever. The effective temperature is 10K for the microcantilever employed in 77K, when the power of the fiber interferemeter is 1 mw. This method could significantly expand the AM mode based dynamic range and force response rate of the microcantilever.