Xiaotun Qiu

Film bulk acoustic-wave resonator based ultraviolet sensor

This letter described ultraviolet (UV) radiation sensing with ZnO based film bulk acoustic-wave resonator (FBAR). The resonant frequency upshifted when there was UV illumination on the FBAR. For 365 nm UV light, the frequency upshift was 9.8 kHz with an intensity of 600 μW/cm2, and the detection limit of the sensor was 6.5 nW. The frequency increase in the FBAR UV sensor was proposed to be due to the density decrease in ZnO film upon UV illumination. When UV was incident on the ZnO film, it can cause oxygen desorption from the ZnO surface, resulting in density decrease in the film. This study has proven the feasibility of detection of low intensity UV using ZnO film based FBAR.

Film Bulk Acoustic-Wave Resonator Based Relative Humidity Sensor Using ZnO Films

This article described relative humidity RH sensing using a ZnO-based film bulk acoustic-wave resonator FBAR . The resonant frequency of the FBAR decreased in a two-stage manner as the RH increased. For low RH, a frequency downshift of 2.2 kHz per 1% RH change was observed. This effect was attributed to water molecules replacing the adsorbed oxygen on the ZnO surface, thus increasing the density of the film. For high RH, a frequency downshift of 8.5 kHz per 1% RH change was obtained, which was due to the mass loading effect of the water layers formed on the ZnO surface.

Localized Parylene-C bonding with reactive multilayer foils

This paper describes a novel bonding technique using reactive multilayer Ni/Al foils as local heat sources to bond Parylene-C layers to another Parylene-C coating on a silicon wafer. Exothermic reactions in Ni/Al reactive multilayer foils were investigated by x-ray diffraction (XRD) and differential scanning calorimetry. XRD measurements showed that the dominant product after exothermic reaction was ordered B2 AlNi compound. The heat of reaction was calculated to be ?57.9?kJ?mol?1. A numerical model was developed to predict the temperature evolution in the parylene layers and silicon wafers during the bonding process. The simulation results revealed that localized heating occurred during the reactive foil joining process. Our experimental observation showed that the parylene layer was torn when the bond was forcefully broken, indicating a strong bond was achieved. Moreover, leakage test in isopropanol alcohol showed that reactive foil bonds can withstand liquid exposure. This study demonstrated the feasibility of reactive foil joining for broad applications in bio-microelectromechanical systems and microfluidic systems.

The effects of relative humidity and reducing gases on the temperature coefficient of resonant frequency of ZnO based film bulk acoustic wave resonator

This study describes the influence of relative humidity (RH) and reducing gases on the temperature coefficient of resonant frequency (TCF) of ZnO-based film bulk acoustic wave resonator (FBAR). Upon exposure to moisture or reducing gases, the TCF of FBAR decreased. Water molecules can replace adsorbed oxygen on the ZnO surface. This process was less effective at high temperature, resulting in a lower TCF in high RH. Reducing gases, such as acetone, can reduce the density of ZnO through reaction with the adsorbed oxygen, leading to a lower TCF.

Film Bulk Acoustic-wave Resonator (FBAR) based humidity sensor

This paper described relative humidity (RH) sensing with ZnO based Film Bulk Acoustic-wave Resonator (FBAR). The resonant frequency of the FBAR decreased in a two-stage manner as the RH increased in the environment. For low RH (RH<50%), a frequency shift of 2.2 kHz per 1% RH change was observed. This effect was attributed to water molecules replacing the adsorbed oxygen on the ZnO surface, thus increasing the density of the film. While for high RH (RH>50%), a frequency shift of 8.5 kHz per 1% RH change was obtained, which was due to the mass loading effect of the water layers formed on the ZnO surface. Ultraviolet (UV) light was applied to monitor its effects on the humidity sensing performance of the FBAR. UV can enhance the sensitivity at low RH (response increased to 3.4 kHz per 1 % RH change), while degrade the sensitivity at high RH (response decreased to 5.7 kHz per 1% RH change). This study has proven the feasibility of measuring relative humidity using ZnO film based FBAR.

Film Bulk Acoustic-wave Resonator (FBAR) based infrared sensor

This paper described an infrared (IR) radiation sensor based on Film Bulk Acoustic-wave Resonator (FBAR). The resonant frequency of FBAR sensor downshifts linearly when there is IR (peak wavelength at 780nm) illumination on the device. This effect attributed to the temperature sensitivity of the FBAR. The noise equivalent temperature difference (NETD) and the detection limit for 780 nm IR of the sensor is 25 mK at 25 °C and 19 μW/mm2, respectively. This study has proven the feasibility of detection of IR using ZnO film based FBAR.

Acetone sensor based on Film Bulk Acoustic Resonator

This paper described acetone sensing using ZnO based Film Bulk Acoustic Resonator (FBAR). The resonant frequency of the FBAR increased as the concentration of acetone increased. The detection limit of acetone was around 4 ppm. The density decrease of the ZnO induced by releasing carbon dioxide generated from the reaction between acetone and the adsorbed oxygen ions on the ZnO surface was assumed to be responsible for the frequency upshift. Upon exposure to ethanol (a major environmental crosstalk for traditional acetone sensors), an opposite response (decrease of resonant frequency) was observed. Water was generated and absorbed on the ZnO surface during the ethanol sensing process. Thus, the density of the ZnO film increased, resulting in a frequency drop. FBAR sensor possessed the unique ability to distinguish acetone from ethanol due to their different reaction behaviors.

A novel technique to cover microfluidic systems with Parylene-C

This paper describes a novel technique for covering microfluidic systems using Parylene-C. Microfluidic systems consisting of micro channels and reservoirs need to be covered to protect or isolate liquid samples from the environment. Thick photoresist and wax are employed as the sacrificial layers in the enclosed micro channels and reservoirs before Parylene-C sealing. The results show that the melted wax improves adherence on a flat and neat Parylene-C film cover and can greatly benefit the mass production. After removing the sacrificial layers, Parylene-C is heated to 120 °C to change the residual stress of Parylene-C film to strongly tensile for a flatter surface.

Film Bulk Acoustic-Wave Resonator based radiation sensor

This paper describes a high-energy electromagnetic wave radiation detection device using zinc oxide (ZnO) based Film Bulk Acoustic-Wave Resonator (FBAR). The resonant frequency of the FBAR decreased after gamma radiation, with the peak sensitivity of 9.3 kHz/krad and minimum detectable dosage of 218 rad occurring at the lowest experimental dose of 20 krad (all dosages are calibrated with ZnO), while the sensitivity decreased with increasing total ionizing dosage. The incident radiation generated charges that was trapped near the ZnO-silicon nitride (SiN) interface, which increased the plate capacitance of the FBAR, resulting in the decrease of resonant frequency.

Reactive multilayer foils for MEMS wafer level packaging

We present a novel room temperature bonding technique using reactive multilayer Ni/Al foils as local heat sources to heat intermediate adhesion layers and thus bond silicon wafers to form wafer level package. Exothermic reactions in Ni/Al multilayer foils were investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). XRD measurements showed that the dominant product after the exothermic reaction was ordered B2 AlNi compound. The heat of reaction was calculated to be −57.9 kJ/mol. Two bonding approaches were realized. One used AuSn solder as an intermediate layer to bond two silicon wafers together; the other approach conducted bonding between Parylene-C and silicon wafers with a thin Parylene-C coating. Either silicon or Parylene-C was torn when the bond was forcefully broken, indicating a strong bond was achieved. Moreover, leakage test in isopropanol alcohol (IPA) showed that the joints possessed good hermeticity to liquid. Numerical simulation results demonstrated that both localized heating and rapid cooling occurred during the bonding process, which made reactive foil bonding an ideal method for MEMS wafer level packaging applications.