The potential of thin film thickness variation measurement method, reflectometric interference spectroscopy (RIfS), for a
compact label-free biosensor is investigated. A model to estimate thickness variation is built based on RIfS. A set-up of
the sensor having dual Light Emitting Diodes (LEDs) and one photo detector are introduced. To verify the model,
sample chips with different thicknesses of silica film layers ranging from 2 to 20nm are used in the experiment. The
estimated values are compared with their reference values which are measured by an Atomic Force Microscopy (AFM).
Since the chosen LEDs' wavelength is not an ideal one, the comparison shows that the model underestimates the
thickness variation. By using dual LEDs and a photo detector with the reliable model, the handheld device for
transparent thin film measurement will become practical.
This paper presents a label-free biosensor using two Light Emitting Diodes (LEDs) as light sources and a photo detector
as a receiver. The sensor uses a silica-on-silicon wafer with PMMA [Poly(methyl methacrylate)] as the functional layer.
The principle of this biosensor is based on the Fabry Perot (FP) interferometer. A thickness of a 100 nm PMMA layer is
spin-coated on the silicon wafer, which has a thin thermal oxide layer of 500 nm. In such a configuration, the PMMA
layer and silica layer function as an FP cavity. When a light illuminates the surface of the sensor, the reflections from the
PMMA-air and silica-silicon interfaces will interfere with each other. Consequently, the change of the cavity length,
which is caused by biomaterial binding on the PMMA layer, will result in a red shift in the reflection spectrum. An
intensity change of the reflection light will be observed on an individual wavelength. In order to eliminate environment
noise and to enhance the sensitivity of the sensor, two LEDs, whose center wavelength is chosen on either side of the
spectrum notch, are introduced in the system. A photo detector will alternatively obtain the intensities of the two
individual reflected lights, and collect the signal via a data acquisition system. Long-term tests have shown that the
sensor is resistant to environmental fluctuation. Biolinker Protein G' was used for binding tests. The sensor shows great
potential in biosensor applications due to its compact size and low cost.
This paper presents a micromachined silicon membrane type AFM tip designed to move nearly 1µm by electrostatic
force. Since the tip can be vibrated in small amplitude with AC voltage input and can be displaced up to 1μm by DC
voltage input, an additional piezo actuator is not required for scanning of submicron features. The micromachined
membrane tips are designed to have 100 kHz ~ 1 MHz resonant frequency. Displacement of the membrane tip is
measured by an optical interferometer using a micromachined diffraction grating on a quartz wafer which is positioned
behind the membrane tip.
A micrograting interferometer has been fabricated to use in measuring the static and dynamic performance of MEMS devices. These measurements aid in qualifying the functionality of fabricated MEMS devices, as well as improving fabrication techniques. The metrology system uses a phase sensitive diffraction grating for interferometric axial resolution and a microfabricated lens for improved lateral resolution. In addition, active control is applied to the system to reduce the impact of mechanical vibrations and insure a high degree of measurement sensitivity. The control scheme is demonstrated successfully in the scanning of MEMS devices in the experiment. A deformable grating, which controls measurement sensitivity, has been fabricated and integrated with optoelectronics in small volume. Experiments with the integrated package demonstrate that the measurement sensitivity can be adjusted by actuating the deformable grating. This integrated single device illustrates that the deformable grating sensor can be expanded to form arrays for parallel measurement of MEMS device.
A micro grating interferometer has been fabricated to use in measuring the dynamic performance of MEMS devices. The system uses a phase sensitive diffraction grating for interferometric axial resolution and a microfabricated lens for improved lateral response. Early experimental results using a non-deformable grating interferometer show that both the transient and steady state vibration of MEMS devices can be measured and mapped using the micro interferometer. These initial results also reveal vibrational noise and sample alignment problems. To avoid these obstacles and to maximize the sensitivity of the interferometer, a PID control unit is introduced. Analysis has been performed on the interferometer system to improve the controller design. A deformable grating interferometer has also been fabricated using microfabrication techniques and tested to show proper range of actuation under DC bias. This grating also demonstrates the ability to maintain a high sensitivity during operation.