Kapila Wijekoon

Development of a production worthy copper CMP process

A chemical mechanical polishing (CMP) process for copper damascene structures has been developed and characterized on a second generation, multiple platen polishing tool. Several formulations of experimental copper slurries containing alumina abrasive particles were evaluated for their selectivity of copper to Ta, TaN and PETEOS films. The extent of copper dishing and oxide erosion of these slurries is investigated with various process parameters such as slurry flow rate, platen speed and wafer pressure. The amount of dishing and erosion is found to be largely dependent on process parameters as well as the slurry composition. It is shown that the extent of oxide erosion and copper dishing can be significantly reduced by using a two slurry copper polish process (one slurry to polish copper and another to polish barrier layers) in conjunction with an optical end-point detection system.

Texture process monitoring in solar cell manufacturing using optical metrology

A novel optical metrology technique has been developed to study textured silicon wafers used to manufacture solar cells. This high efficiency optical design to maximize the signal from surfaces with reflectivity well below 1% was developed. Pyramid dimensions were measured on as sawed p-type Czochralski wafers having a bulk resistivity of 1–5 Ohm-cm. These wafers were subjected to a single step texturization process by using a non-alcoholic chemical etching formulation. Results were compared with SEM imaging. Reflectivity tests and pyramid height measurements were used to study the efficiency of the texturing processes. In a related study [2] it was found that 20μm of material removal was required to attain minimum surface reflectivity. Further removal of material affected pyramid dimensions, but did not improve surface reflectivity.

Optimization of Rear Local Contacts on High Efficiency PERC Solar Cells Structures

A local contact formation process and integration scheme have been developed for the fabrication of rear passivated point contact solar cells. Conversion efficiency of 19.6% was achieved using  mm, 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 18.9% conversion efficiency on the same wafer type. The effect of rear contact formation on cell efficiency was studied as a function of contact area and contact pitch, hence the metallization fraction. Contact shape and the thickness of Al-BSF layer were found to be heavily dependent on the laser ablation pattern and contact area. Simulated cell parameters as a function of metallization showed that there is a tradeoff between open circuit voltage and fill factor gains as the metallization fraction varies. The rear surface was passivated with an Al2O3 layer and a capping layer. 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. The efficiency gain in rear passivated cells over the reference cells is mainly due to improved short circuit current and open circuit voltage.

Direct texturization of as sawed mono-crystalline silicon solar wafers: Solar cell efficiency as a function of total silicon removal

Sawed p-type Czochralski wafers with a bulk resistivity of 1–5ωcm were subjected to single step texturization process by using a non-alcoholic chemical etching formulation. The minimum amount of silicon removal required for achieving optimum solar cell efficiencies was estimated by measuring cell efficiency as a function of silicon loss. The surface damage layer of as cut wafers was found to be approximately 6μm deep as indicated by the minority carrier lifetime measurements. However for attaining minimum surface reflectivity it was required to remove at least 20μm of silicon from the initial wafer. Further removal of silicon offered no additional improvements in surface reflectivity or the solar cell efficiency.

Experimental and theoretical analysis of the optical behavior of textured silicon wafers

Optical analysis is performed for mono-crystalline silicon wafers with and without the texture and with and without the POCl doping, the passivating anti-reflective nitride film on front surface, and the screen printed aluminum conductor on the back surface. Reflectance is measured in the wavelength range from 300 nm to 1200 nm. Modeling of the light reflectance, absorbance, and transmittance is done using ray-tracing technique for the regular and the random texture patterns. Good agreement of measured and modeled data is obtained for the sub — 1 micron wavelengths by using standard material optical properties. However, the infrared light above the 1 micron wavelength requires accounting for several mono-layers thick native oxide present on silicon surfaces and adjusting the optical properties of specific nitride and aluminum films used in the solar cell manufacturing. It is found that the random texture exhibits 15% to 20% better light capture than the regular texture. Theoretical analysis provides plausible explanation of this effect and suggests a way to further improve optical performance of the textured surfaces. The optical modeling methodology can be used to find the optimum combination of texture and passivating/contact films for different solar cell designs.

Production ready noval texture etching process for fabrication of single crystalline silicon solar cells

We have very successfully developed a novel, IPA free chemical etching process for texturing single crystalline silicon substrates by employing polymer additives with aqueous KOH. In this paper we describe the successful implementation of this IPA free novel texturization process in our baseline single crystalline solar cell flow. The results have been validated with a number of repeated extended run production data. Solar cells fabricated with multiple wafer lots consistently exhibit cell efficiencies greater than 17.5%. It is found that surface texturing capability of this chemistry is slightly better than that of the established KOH/IPA process

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.

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%.

Polishing and Cleaning of Low K Dielectric Material for ild and Damascene

This paper describes CMP challenges in development of copper-low k process technology. As copper/oxide or copper/FSG backend schemes are being implemented successfully in early manufacturing, development focus has shifted to Cu/OSG (organo-silicate glass) integration development. Cu-OSG presents unique challenges with CMP integration, as these films tend to have much lower hardness than silicon dioxide. Significant process challenges have to be overcome prior to successfully implementing CMP process which does not mechanically damage the softer films and at the same time can achieve planarization requirements expected from CMP process. In addition, the OSG films tend to be hydrophobic leading to a need for developing improved cleaning processes/consumables. It was determined that Applied Materials ElectraPolishTM barrier slurry is extendable to OSG films. Good removal rate and removal profile can be achieved with ElectraPolishTM slurry. A proprietary cleaning solution reduced defect counts by 2 orders of magnitude as detected by SurfScan SS6200 on blanket OSG wafers. The same cleaning solution can be applied to copper/low-k patterned damascene wafers to clean both copper and dielectric surface. Polished OSG films have RMS roughness less than 2 angstroms and copper surface roughness about 5 angstroms with good surface finish. Blanket and patterned wafer results are presented to demonstrate final capability. Future directions for process enhancement are suggested.