Abhijat Goyal

Tin-based solder bonding for MEMS fabrication and packaging applications

This paper presents a tin-based solder bonding technology for microelectromechanical systems (MEMS) fabrication and packaging applications. Electroplated tin on a lithographically patterned seed layer of chrome–gold was used for bonding two Pyrex substrates. Average values of the tensile strength and the shear strength obtained were 14.6 MPa and 5.78 MPa, respectively. Shear strength data were analysed using Weibull statistics, which revealed presence of multiple failure mechanisms and an average value of β greater than 3. Failure occurred not only at the tin–tin bond interface but also at the tin–chrome interface. Highest values for the bond strength were obtained when the samples failed due to delamination at the tin–tin bond interface. Acoustic images of the bonded interface were used to qualitatively study the degradation in the bond interface when subjected to tensile stresses. Hermetic testing of the samples using a conventional He-leak detector showed leak integrity of better than 1 × 10−11 mbar l s−1.

High aspect ratio plasma etching of bulk lead zirconate titanate

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.

Fabrication and performance of a flextensional microactuator

A flextensional microactuator has been designed and fabricated for the amplification of the small strain of the piezoelectric materials to achieve large displacements. Bulk PZT material available in the form of 500 µm thick polished substrate has been integrated with a precision micromachined silicon beam structure to achieve the clamped–clamped flextensional microactuator. A high strength, high precision (alignment) and low temperature (~200 °C) In/Sn solder bonding process has been developed and used for the fabrication of the flextensional microactuators. Actuators with physical dimensions of the flextensional structure ranging from 350 to 600 µm in length, 50 to 100 µm in width and 5 to 6 µm in thickness were fabricated. The measured static deflection characteristics of the silicon micromachined beam show a flextensional gain factor of 20 with a large amplitude stroke of ~8 µm when actuated using −100 V to 100 V. The bandwidth of the actuator was experimentally measured to be 265 kHz. The fabricated devices show good repeatability with a hysteresis pattern arising from the PZT characteristics. The bonding technique described here can be used for the precision integration of heterogeneous materials for MEMS device fabrication and their packaging.

Use of single-walled carbon nanotubes to increase the quality factor of an AT-cut micromachined quartz resonator

In this letter, we report the suppression of loss mechanisms in an AT-cut quartz resonator operating in thickness shear mode using an over layer of single-walled carbon nanotubes (SWNTs), and the resulting increase in the Q (quality) factor of the resonator by as much as 100%. The Q factor was found to monotonically increase as more SWNTs were added to the resonator. The increase in the Q factor of the resonators is thought to arise due to suppression of surface loss modes due to interaction of the carbon nanotubes with the quartz resonator surface. The use of SWNTs provides a very effective and simple way to improve the performance of quartz resonators.

Reliable bonding using indium-based solders

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.

Solder bonding for microelectricalmechanical systems (MEMS) applications

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.

Y-cut quartz resonator based calorimetric sensor

Monitoring of chemical and biochemical reactions such as neutralization reactions, antibody-antigen binding events, and enzyme catalyzed reactions, etc. can be achieved using calorimetric (bio)chemical sensors. A calorimeter array consisting of a thin film thermopile as temperature sensor integrated with microfluidic channels has already been demonstrated recently. In order to improve the sensitivity of the device, a temperature sensing element based on Y-cut quartz is proposed. The temperature-frequency calibration of a 125 mum thick Y-cut quartz resonator has been experimentally measured in the 22-60degC temperature range. Preliminary measurements of the neutralization reaction have been performed using the resonator to demonstrate the potential of the device for calorimetric sensing applications. It can be concluded that upon optimization of the device for thermal performance and appropriate compensation of mass loading and viscoelastic effects using a differential arrangement, ultrasensitive calorimetric measurements can be performed using such a device

Improvement in Q-factor of AT-cut quartz crystal resonators using single walled carbon nanotubes

Use of higher operating frequencies and integration with VLSI circuits and MEMS are driving the need for smaller and thinner quartz crystal resonators (QCRs). Implicit in such scaling is the maintenance of Q-factor of the resonator necessary for achieving the required frequency stability. The intrinsic Q- factor of the resonator is inversely proportional to the resonance frequency, limiting the maximum Q that can be achieved. Recent research in this field has been directed towards increasing the Q- factor of these resonators, including reduction of surface roughness, decreasing support losses by reduction of resonator thickness, making resonator surface convex to increase energy trapping, etc. In this paper we report the first observation of the increase in the quality factor of an AT-cut quartz resonator through deposition of thin layers of single walled carbon nanotubes on its electrodes.

Micromachined quartz resonator functionalized with single walled carbon nanotubes

Single walled carbon nanotubes (SWNTs) are reservoirs of gases as they can adsorb on their walls as well as retain gas molecules in their hollow cylindrical interior. This has important applications for example in fuel cell technology for hydrogen storage, as a gas sensor for realization of artificial nose, etc. Storage of gases by carbon nanotubes have been recently investigated by monitoring changes in their thermoelectric power and electrical resistivity due to their interaction with gas molecules. In this paper we present a gravimetric study of interaction of gas molecules with isolated SWNTs using a micromachined ultrasensitive quartz crystal microbalance (QCM). The adsorption and desorption of gas molecules with different molecular weights from carbon nanotubes revealed that changes in resonance frequency and quality factor of the resonating crystal scale as approximately M0.45, where M is the mass the of the gas molecule as compared to M1/3 dependence observed in case of changes in thermoelectric power and electrical resistance for thin films of the carbon nanotubes. The use of QCM enables room temperature probing of gas interaction with isolated single walled carbon nanotubes. Specific interaction of gases with carbon nanotubes on QCM provides potential application of the device as a gas sensor

SAMs and Protein Adsorption Studies for the Calibration of Miniaturized Quartz Crystal Microbalance Arrays

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 mum 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 1364 over a commercially available macro-sized QCM. In both rigid and viscoelastic film adsorption limits, 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.