Lensless multi-spectral holographic interferometry for optical inspection

We explore the principles, implementation details, and performance characteristics of a lensless multi-spectral digital holographic sensor and demonstrate its potential for quality assurance in semiconductor manufacturing. The method is based on capturing multi-spectral digital holograms, which are subsequently utilized to evaluate the shape of a reflective test object. It allows for a compact setup satisfying high demands regarding robustness against mechanical vibrations and thus overcomes limitations associated with conventional optical inspection setups associated with lens-based white light interferometry. Additionally, the tunable laser source enhances the versatility of the system and enables adaptation to various sample characteristics. Experimental results based on a wafer test specimen demonstrate the effectiveness of the method. The axial resolution of the sensor is <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m1"><mml:mo>±</mml:mo><mml:mn>2.5</mml:mn></mml:math> nm, corresponding to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m2"><mml:mn>1</mml:mn><mml:mi>σ</mml:mi></mml:math> .