We report the design and fabrication of a micromachined quartz crystal balance (QCM) array for self
assembled monolayers (SAMs) and protein adsorption studies. The microQCM was fabricated using recently
developed inductively coupled plasma etching process for quartz to realize resonators with 60 &mgr;m thickness
and electrode diameters of 0.5 mm. The reduction in the thickness and lateral pixel size has resulted in a
sensitivity improvement by factor of 1700 over a commercially available macro-sized QCM. Adsorption of
hexadecanethiol on the gold electrode of the QCM in ethanol at a concentration of 1 mM was recorded in real
time and a frequency shift of 3650 Hz was obtained. Modeling the SAMs layer as an ideal, rigid mass layer
the expected frequency shift was calculated to be 1031 Hz. This was followed by a study of the adsorption of
human serum albumin (HSA) protein on the SAMs layer. For 1.5×10-10 moles/ml concentration of protein
solution in phosphate buffer solution (PBS) we obtained a frequency change of 13.28 kHz. Modeling the
protein layer as a viscoelastic layer in a viscous Newtonian fluid, for saturation protein surface coverage, the
frequency change was calculated to be 17.27 kHz whereas the experimentally obtained frequency change was
51.82 kHz. In both rigid and viscoelastic film adsorption experiments, we find the microQCM to exhibit three
times greater sensitivity than the predicted value when operated at the third overtone. These results show that
the micromachined QCM in array format is a very sensitive gravimetric sensor capable of mass resolutions
into the femtograms range.
Lead Zirconate Titanate (PZT) is a high energy density active material with good piezoelectric coefficient and electromechanical coupling constant making it highly suitable for microsystems applications. In this paper, we present a rapid anisotropic high aspect ratio etching process for defining micron size features in PZT. We used an inductively coupled plasma reactive ion etching (ICP-RIE) system employing sulfur hexafluoride (SF6) and argon (Ar) based chemistry. A seed layer of Au/Cr was lithographically patterned onto fine lap finished PZT-4 substrates followed by electrodeposition of a thick 2-5 μm nickel on the seed layer, which acts as a hard mask during the etching process. The demonstrated technique was used to etch bulk PZT ceramic substrates, thereby opening possibilities for integration of bulk PZT substrates and structures into microsystems. A maximum etch rate of 19 μm/hr on PZT-4 and 25 μm/hr for PZT-5A compositions was obtained using 2000 W of ICP power, 475 W of substrate power, 5 sccm of SF6, and 50 sccm of Ar on PZT substrate. We have also demonstrated a high aspect ratio etch (>5:1) on a 3 μm feature size. Detailed analysis of the effects of ICP power, substrate power, and the etch gas composition on the etch rate of PZT are also presented in this article.
It has been shown that the addition of single walled carbon anotubes (SWNTs) cause an increase in the resonance frequency of micromachined clamped-clamped structures. This is believed to be due to an increase in the effective stiffness of the micromachined structures due to the high Young's modulus of carbon nanotubes. These results were obtained in spite of a relatively poor control over the orientation and aerial density of the deposited SWNTs. Finite element simulations showed an increase in the resonance frequency of up to ~25% for the simulated devices. This increase in the resonance frequency of the bridges can be attributed to the high Young's modulus (~1TPa) of the carbon nanotubes.
Etching of quartz and glass for microsystems applications requires optimization of the etch process for high etch rates, high aspect ratios and low rms surface roughness of the etched features. Typically, minimum surface roughness of the etched feature accompanied with maximum etch rate and anisotropy are desired. In this article, we investigate the effect of different gas chemistries on the etch rate and rms surface roughness of the Pyrex(R) 7740 in an inductively coupled plasma reactive ion etching (ICP-RIE) system. The gases considered were SF6 and c-C4F8, with additives gases comprising of O2, Ar, and CH4. A standard factorial design of experiment (DOE) methodology was used for finding the effect of variation of process parameters on the etch rate and rms surface roughness. By use of 2000 W of ICP power, 475 W of substrate power, SF6 flow rate of 5 sccm, Ar flow rate of 50 sccm, substrate holder temperature of 20°C, and distance of substrate holder from ICP source to be 120 mm, we were able to obtain an etch rate of 0.536 μm/min and a rms surface roughness of ~1.97 nm. For an etch process optimized for high etch rate and minimum surface roughness using C4F8/SF6/O2/Ar gases, an etch rate of 0.55 μm/min and a rms surface roughness of ~25 nm was obtained for SF6 flow rate of 5 sccm, C4F8 flow rate of 5 sccm, O2 flow rate of 50 sccm, Ar flow rate of 50 sccm. Keeping all other process parameters the same, increasing the SF6 flow rate to 50 sccm resulted in an etch rate of 0.7 μm/min at an rms surface roughness of ~800 nm whereas increasing the C4F8 flow rate to 50 sccm resulted in an etch rate of 0.67 μm/min at an rms surface roughness of ~450 nm . Addition of CH4 did not contribute significantly to the etch rate while at the same time causing significant increase in the rms surface roughness. Regression or least square fit was used define an arbitrary etch rate number (Wetch) and rms surface roughness number (Wrms). These numbers were calculated by least square fit to the data comprising of ten correlated etch variables and enable quantization of etch parameters in terms of process parameters. The etch numbers defined in this work as function of process parameters present a very useful tool for the optimization, quantification and characterization of the dielectric etch processes developed in this work for MEMS fabrication and packaging applications.
Low temperature bonding techniques with high bond strengths and reliability are required for the fabrication and packaging of MEMS devices. Indium and indium-tin based bonding processes are explored for the fabrication of a flextensional MEMS actuator, which requires the integration of lead zirconate titanate (PZT) substrate with a silicon micromachined structure at low temperatures. The developed technique can be used either for wafer or chip level bonding. The lithographic steps used for the patterning and delineation of the seed layer limit the resolution of this technique. Using this technique, reliable bonds were achieved at a temperature of 200°C. The bonds yielded an average tensile strength of 5.41 MPa and 7.38 MPa for samples using indium and indium-tin alloy solders as the intermediate bonding layers respectively. The bonds (with line width of 100 microns) showed hermetic sealing capability of better than 10-11 mbar-l/s when tested using a commercial helium leak tester.
MEMS fabrication and packaging requires a bonding technology that is universal for all substrates, has high resolution, requires relatively lower temperatures, is reliable and is low cost to implement. The bonding technology presented meets the above standards. The process is substrate independent and involves aligned bonding of two similarly patterned wafers using tin solder as the bonding material. The technique can be used for whole wafer or selected area bonding. The resolution of this technique is only limited by the resolution that can be achieved in the patterning and delineation of the seed metal.