High aspect ratio contacts: A review of the current tungsten plug process

Although all areas of semiconductor manufacturing have experienced the reality of miniaturization, this review is intended to highlight the key materials and processing parameters for successful high aspect ratio contacts (HARCs) in the tungsten plug process. Seven fundamental points of interest are highlighted: (1) dopant levels, (2) cleans, (3) contact layer, (4) barrier layer, (5) Ti-TiN anneal, (6) tungsten fill, and (7) tungsten etchback. Several of these factors-residual native oxides, Ti thickness at the bottom of the contact, and anneal temperature-toggle the amount of silicidation at the bottom of the contact in an interactive way and need to be optimized accordingly along with the W seed layer to minimize TiFx, formation during tungsten deposition. Other detrimental contact-related defects are discussed with emphasis on their source. Ti cusping and grain growth as well as TiN and W film conformality are shown to be first order factors in forming W keyholes. Collimation has been a means of minimizing cusping and achieving sufficient Ti at the bottom of the contact, but the logistics of increasing collimation for that required for 5:1 HARCs and beyond (deposition rates and collimator changes) may provide an opportunity for one of the newer technologies. TiN has been shown to act as a barrier to W wormholes. Two means of depositing TiN are presented with their inherent advantages and disadvantages. Sputter-deposited TiN is a low resistance form of TiN, but does not deposit on the sidewalls of the contact. The more conformal chemical vapor deposited (CVD) TiN process provides a more complete barrier with better sidewall coverage, but has higher sheet resistance. Tungsten planarization can be accomplished by dry etch or chemical-mechanical polishing (CMP). Dry etch is effective in removing W but ineffective in removing the large W defects that are often associated with the deposition process. CMP more readily planarizes the larger defects, but is more likely to scratch the oxide. Optimization of each is required. (C) 1997 Elsevier Science S.A.