UV Cure Enabled Robust STI Oxide Gap Fill Solution for Thermally Limited Flow in Advanced Logic Devices
Authors: Mehta Sanjay C , Conti Richard , Devarajan Thamarai , Wright Matthew P , Yao Yiping , Cohen Stephan A , Ryan Todd , Haigh Thomas J , Hall Paul , Canaperi Donald F , Tai Leo 
Published: February 2020

High mobility channel materials such as Si1-xGex (x = 0.1-0.5) are important to achieve performance targets for FinFET devices at dimensions scaled beyond 14nm. Previously, flowable (FCVD) SiO2 deposition processes have been developed to achieve adequate fill for Shallow Trench Isolation in FinFET structures. However, the thermal budget for curing FCVD oxide with SiGe fins must be limited to avoid out-diffusion of Ge. This restricts the processing options for achieving a robust material which is compatible with subsequent wets processing steps.
FCVD oxide deposition employs an amine-based silicon precursor which, upon exposure to NH3 radicals, forms short chain -Si-N-Si- oligomers with terminal Si-H or N-H bonds. These oligomers flow into and fill deep isolation trenches. Once the trenches are filled, it is critical to stabilize the film and initiate cross linking for densification to SiO2. To this end, a cure while still under vacuum, such as O3 oxidation, is commonly used before a subsequent furnace steam anneal. This paper describes an alternate curing process using broadband UV light to densify FCVD oxide. Material properties were studied using cure by microwave-powered Hg lamps at 10 degC with broadband (200-450nm) spectrum and compared to the existing O3 cure.
To achieve the desired FCVD film properties, it is beneficial to remove -OH terminations during the post-dep cure to promote a dense final -Si-O-Si- matrix. Fig. 1 shows high frequency FTIR spectra from UV-cured FCVD oxide films as a function of UV power post-steam anneal. Data from O3 cured FCVD oxide is plotted for comparison. The spectrum from the non-oxidizing UV cure process shows lower peak intensities for –OH and H-OH stretch peaks. For promoting densification, UV curing is also effective in breaking N-H bonds. The number of NH/NH2 bonds is reduced with increasing UV power and a corresponding increase in Si-H bonding is observed. Figure 2 shows FTIR spectra of cured FCVD films treated with a subsequent steam anneal. The film with O3cure show more residual silanols (Si-OH) relative to UV cured films. Also, the -Si-O-Si- bonding peak is higher in UV cured films, suggesting a denser -Si-O-Si- network.
During the steam anneal process, we observed that UV cured films densify more rapidly than O3 cured films. For anneals at 500°C, a dense SiO2 surface layer is formed, blocking further O2diffusion and leaving a less dense film below which has a higher wet etch rate (Fig. 3). This effect is evident in Fig. 4a where FCVD oxide has been recessed using an etching technique. The oxide between fins is weaker and has recessed more than the oxide in the field region outside of the array (see reference lines). A two-step steam anneal was then developed to achieve uniform properties through film depth: 400°C for 1 hour followed by 500°C for 2 hours. Fig 4b shows a structure with oxide annealed in this way and then recessed through etching; it was observed that the recess is now equivalent in the field and between fins.
The impact of UV power on the cure process was also explored. Fig. 5 shows the WERR as a function of thickness for blanket O3 cured FCVD films and UV cured films after steam anneal. The UV cure again shows significantly lower WERR compared to O3 cure, with more densification and improved film uniformity through depth with increased power. Fig. 6 compares cumulative electrical breakdown behavior for O3 cured and UV cured films after steam anneal obtained using MIS structures. The UV cured films show a higher breakdown voltage (Vbd ~10MV/cm) relative to O3 cured films (~9MV/cm). Fig. 7 is a comparison of WERR between O3 cured film and UV cured film inside the narrow STI Trench Features. The films were steam annealed followed by 2x900°C-1min soak RTA in N2. Data shows 30% lower in-trench WERR for UV cure compared to O3cure.
In conclusion, UV cure has been demonstrated as a means to provide improved FCVD quality over O3cure as measured by wet etch rate ratio, in blanket films and within trenches for STI applications.

This work was performed by the Research Alliance Teams at various IBM Research and Development Facilities.

Figure 1