Non-Destructive Metrology Techniques for Characterizing a-C Hard Mask Films in 3D NAND Structures

In this paper, we present results from successful application of two non-destructive techniques: Picosecond Ultrasonics and FTIR for the qualification of next generation of advanced hard mask materials.  A design-of-experiments (DOE) skew (~3µm a-C thickness target) covering different deposition temperature, pressure and time were generated.  Picosecond ultrasonic technology is well-established as in-line metrology tool-of-record in wafer fabs for metal films metrology.  With the transition of a-C to thicker and more graphitic (opaque) films, the PULSE™ technique is uniquely qualified for measuring not only the thickness but also the elastic modulus of these films.   Results from the systematic study of different types of films demonstrated sensitivity to process variation and excellent correlation to cross-section microscopy.  Since the measurements take only a few seconds per site, within wafer uniformity information can be collected easily and used to optimize the process thus reducing development time.  

 A second set of a-C films in the 0.2-2µm thickness range was generated to test sensitivity to different processes and provide information regarding the bonding characteristics.  Studies have shown the correlation between structural characteristics and the etching characteristics which is primarily determined by the sp3/sp2 ratio.   Wafer maps have been collected using an Element™ 300mm automated FTIR. We observed that the absorbances for the sp² c=c and the sp³ C-Hx stretching bonds were significantly different and could be used to discriminate the processes. Furthermore, the sp2/sp3 ratio was successfully calculated for all samples. Current efforts are focused on finding correlation between the two metrology techniques and their application in etch process optimization for high volume manufacturing.