248 nm photolithography compatibility on low-k dielectrics in BEOL interconnects

摘要

Printing small geometries using wavelength of 248 nm on low-k dielectric materials is not a plug-in photolithography process from one technology to other technology node. In this paper, a method of film characterization of low-k dielectric materials will be discussed. For a characterization of chemical vapor deposited (CVD) low-k (k-2.8) dielectric materials, a positive tone deep ultra violet (DUV) chemically amplified photoresist (CAR) was used as a poisoning gauge. In early development stage of low-k dielectrics and copper dual damascene interconnects in back-end-of-line (BEOL) processes, unstable pattering behaviors were observed in spite of using an organic bottom antireflective coating (BARC) layers on low-k film substrates. The initial work was focused on finding the source of lot-to-lot critical dimension (CD) variations and understanding what causes this problem as well. Study indicated a strong correlation that photo CD depended on time interval between photolithography process and previous process step. Significant photo CD shift was introduced by short cycle time from thin film deposition to photolithography process and post via etch clean process to trench photolithography process. To minimize photo CD variations, the process optimizations were necessary in low-k dielectric film deposition, rework, via etch process, and post via etch clean process. As parallel efforts to improve lot-to-lot CD control, various photoresist system different ambient annealing conditions, various surface organic and inorganic capping techniques were tested. In this experiment, time interval between processes was tightly controlled and maximized the worst case of scenario. Fresh and aged low-k dielectric films were analyzed using time-of-flight secondary ion mass spectrometry (TOF SIMS) and X-ray photoelectron spectroscopy (XPS) techniques. This work suggested that N2 containing in the film or introducing N2 into low-k dielectric film caused lot-to-lot photo CD variations.