Proximity bias swing: origin and characterization

Abstract

Isolated to dense linewidth offsets, also known as proximity bias, can consume a significant portion of the CD budget. As a result, it has received great attention over the recent years. It is demonstrated that proximity bias shows a cyclic swing behavior on reflective substrates with respect to resist thickness variations. The amplitude of proximity bias swing was found to be influenced by coherence, substrate reflectivity, feature dimension and pitch. Proximity bias swing is caused by differences in optical path lengths of light passing through the resist film. Due to different diffraction angles for different pitches, the incoupling positions for different pitches vary. The offset in CD swing incoupling positions for different pitches results in proximity bias swing. At low coherence however, an averaging effect on diffraction angles from different pitches takes place due to the wide range of angles of light passing through the mask. In addition, the impact of exposure margin variations on resolution and proximity bias was demonstrated. Low exposure margins offer high resolution. As a consequence, high proximity bias is observed. Furthermore, different line to space ratios were studied to identify the transition point between dense and isolated features with respect to proximity bias swing. At high coherence ((sigma) equals 0.35) it was observed that for 0.25 micrometers features with pitches smaller than 0.65 micrometers , proximity bias swing is larger than the +/- 0.5% CD budget, which makes it impossible to do effective application of proximity bias correction schemes. At low coherence, only limited proximity bias swing was found. Through variation of bake conditions it was demonstrated that these process variations had no measurable effect on proximity bias swing. Optical settings, in combination with substrate reflectivity, are the main contributors to (eliminate) proximity bias swing.