Lambda Theta Reflectometry: a new technique to measure optical film thickness applied to planar protein arrays

Quantitative protein measurements provide valuable information about biological pathways, immune system functionality, and mechanisms of disease. The most accurate methods for detecting proteins are label-free and preserve native protein binding interactions. Label-free biomolecular interaction analysis includes reflectometry, a group of techniques that detect proteins by measuring the reflectance properties of a thin film on a substrate. Most of these techniques are limited in some way by instrument complexity, sensitivity, or consumable manufacturing requirements. To address these issues, we introduce Lambda Theta Reflectometry (LTR), a new reflectometric technique that measures changes in film thickness by determining the location of null reflectivity as a function of wavelength (lambda) and angle of incidence (theta). The substrate is simultaneously illuminated with a range of angles and wavelengths and reflected light is angularly and spectrally resolved. Our prototype LTR reflectometer can measure SiO₂ layer thickness with milli-Ångstrom precision. LTR measurements of Si/SiO₂ oxide films are in excellent agreement with spectroscopic ellipsometry for film thicknesses ranging from 1390-1465 A. This technique enables sensitive measurements across a range of biological analyte concentrations without requiring stringent control over probe deposition thickness or substrate manufacturing.