Manoj Vellaikal

Development of high efficiency mono-crystalline silicon solar cells: Optimization of rear local contacts formation on dielectrically passivated surfaces

An integration process was developed for the fabrication of rear passivated point contact solar cells achieving 19.36% conversion efficiency by using 156×156mm, pseudo square, p-type single crystalline silicon wafers. This is a significant improvement when compared to unpassivated, full area aluminum back surface field solar cells, which exhibit only 18.64% conversion efficiency on the same wafer type. The rear surface was passivated with a Al2O3 layer and a SiNX capping layer. The thicknesses of individual films were optimized to obtain maximum minority carrier lifetimes. The rear surface contact pattern was created by laser ablation and the contact geometry was optimized to obtain voids free contact filling resulting in a uniform back surface field. Internal quantum efficiency and reflectance measurement show significant improvement in rear passivated cells in the infrared wavelength region in comparison to reference cells. The rear surface internal reflectivity for the passivated cell was 93% while that for the reference cell was only about 73%. The rear surface recombination velocity for the rear passivated cell was about 52 cm/s while that for the reference cell was about 300 cm/s. The efficiency gain in rear passivated cells over the reference cells is mainly due to improved short circuit current and open circuit voltage. However, rear passivated solar cells show lower fill factors due to increased series resistance.

Extending HDP for STI fill to 45nm with IPM

A novel gap fill approach, Integrated Profile Modulation (IPM), with repeating deposition and etch cycles is developed to extend HDP for complete gap fill to 45 nm node and beyond, with established HDP CVD film properties and integration. In this paper, the key aspects of the IPM process, deposition, etch, in-film fluorine and aluminum control are discussed to provide better understanding on gap fill optimization and manufacturing capability.

HDP-FSG Integration in Multilevel Interconnect Devices

HDP-FSG has been integrated as an inter-metal dielectric in a multilevel interconnect scheme. Process regimes for obtaining stable HDP-FSG films were identified. Gap-fill of high aspect ratio structures was achieved for 0.18µm technology node. HDP-FSG film stability, homogeneity, and impurity content were evaluated. In addition to the development of the HDP-FSG process, integration of the interconnect module was investigated and optimized. Analysis of the AI/FSG interface integrity was tested for gap-fill and corrosion with SEM and for fluorine diffusion with SIMS. CMP rate of the FSG was determined and compared to HDP-USG. Al metallization integration approaches in conjunction with FSG were identified.

Progress in production-worthy point contact solar cells process

This work reports on the integration process of rear point contact solar cells with reduced recombination and better light trapping than the conventional cells. Al₂O₃/SiN_x passivation stacks were used to ensure the backside passivation and the effective lifetime of minority carrier is found to be >;100 μs (estimated surface recombination velocity ~50 cm/s) on solar p-type Cz wafers. The estimated fixed charge associated with the Al₂O₃ is in the range of -5e12 C/cm². Laser ablation of Al₂O₃/SiN_x stack and aluminum alloying are studied in detail to understand the process window for clean ablation and back surface field. It is found that laser energy plays an important role cleanly ablating the Al₂O₃/SiN_x passivation stack. The solar cell is fabricated using standard processes based on screen-printed aluminum paste onto laser ablated passivation layer consisting of Al₂O₃/SiN_x. Aluminum alloying is dependent on the firing profile and amount of Al melted into Si. Back point contact cells show improvements in J_sc by 1 mA/cm² and V_oc by 10 mV due to better response in infrared spectrum. The best conversion efficiency of back point contact solar cells fabricated with standard industrial emitter and backside passivation is 19.35%.