On the blistering of atomic layer deposited Al2O3 as Si surface passivation

This work proves that blistering is the partial delamination of a thick enough Al₂O₃ layer caused by gaseous desorption in the Al₂O₃ layer upon thermal treatments above a critical temperature: the Al₂O₃ layer acts as a gas barrier and bubble formation occurs. First, using an atmospheric pressure rapid thermal processor with an atmospheric pressure ionization mass spectrometry, desorbing species upon heating of Si/Al₂O₃ samples are identified: evident desorption peaks are observed around 400 °C for all spectra. The spectrum for m/e = 18, an indication of H₂O, illustrates that gaseous desorption from Al₂O₃ and from the Si substrate itself continues up to 600 °C and 700 °C, respectively. Also, it is shown that in the case of a 30 nm Al₂O₃ layer, blistering starts at same annealing temperatures as gaseous desorption begins. In the case of a thin enough (<; 10 nm) Al₂O₃ film, blistering does not show. To complete the proof, elastic recoil detection measurements clearly show that after annealing a thick Al₂O₃ film above 400 °C the H content is higher near the c-Si interface as compared to the near surface. Fortunately, effective lifetime and capacitance voltage measurements show that 5 to 10 nm Al₂O₃ layers can still be adequate passivation layers after being annealed in N₂ environment at temperatures up to 500-700 °C: (i) interface trap densities (D_it) can remain below 1×10¹¹ cm^-2 and (ii) fixed charge densities (Q_f) stay negative and in the order of -3×10¹² cm^-2 Random local Al back surface field (BSF) solar cells, fabricated using a blistered film as rear surface passivation and no additional contact opening step, clearly show t- at random local BSFs are created upon firing of a blistered rear passivation layer covered by metal. Therefore, it is clear that blistering should be avoided, since it will reduce the overall rear surface passivation. The key to avoid blistering is using 5 to 10 nm Al₂O₃ passivation layers and performing an annealing step prior to capping and co-firing. Al₂O₃/SiN_x passivated local Al BSF p-type Si solar cells are made using an out-gassing step with temperatures up to 700 °C. For these cells, the reduction in blistering and hence improvement in rear surface passivation is clearly reflected in the gain in average Voc as a function of out-gassing temperature.