Chemical Mechanical Polishing of Stainless Steel as Solar Cell Substrate

Abstract

Recently, stainless steel has been widely used as the solar cell substrate. In this paper, chemical mechanical polishing technique was employed to prepare the ultra-smooth 316L stainless steel surface for such application. The effects of solid content, pH, H 2 O 2 and benzotriazole (BTA) on the polishing performance of 316L stainless steel were investigated. The results indicated that, at pH 4.00, with the increase in the H 2 O 2 concentration, the MRR first dramatically increases due to the fact that, with the addition of small amount of H 2 O 2 , a porous outer layer mainly consisting of iron-enriched oxides with relatively low mechanical strength of the bilayer-structure oxide film is rapidly formed on the surface. After reaching the maximum value, the MRR gradually decreases since the outer layer gradually grows compact by the transformation of γ-FeOOH into α-FeOOH and even into α-Fe 2 O 3 . With the addition of BTA, the MRR can be suppressed, which is probably attributed to the formation of BTA passivating film on the top surface. Finally, a two-step polishing method was proposed, by which the ultra-smooth 316L stainless steel surface with the surface roughness R a about 2.0 nm can be achieved within 30 min and it can be subsequently used for thin-film solar cells. Stainless steel plays an important role in the mechanical industry due to the excellent mechanical properties, the high thermostability and the strong corrosion resistance. Recently, various types of stainless steels have been intensively used in precise devices, 1-4 especially been used as the substrate for thin-film solar cells. [5][6][7][8][9][10] In order to achieve satisfactory performance of solar cells, low roughness, low defects as well as excellent flatness of the stainless steel substrate are intensively demanded and even indispensable. It is reported that, by improving the surface roughness of stainless steel substrate from 38 nm to 23 nm, the conversion efficiency of hydrogenated amorphous silicon thin-film solar cells can greatly increase to 5.4%. 10 As is revealed, chemical mechanical polishing (CMP) combines the synergetic effects of chemical corrosion and mechanical abrasion, and can achieve both local and global planarization of the substrate surface. 11-14Hu et al.2 used colloidal silica as the abrasive and investigated the effects of pH and oxidizer on the polishing performance of stainless steel, and it was reported that the combination of oxidizer and strong acidity was the prerequisite for high material removal rate (MRR). However, the microscopic defects of 1-2 μm in size cannot be effectively avoided on the polished surface. Lee et al.10 used electrochemical mechanical polishing technique, which applied an additional anodic potential on the stainless steel substrate besides CMP, to polish the stainless steel surface, and it was reported that the polished surface roughness was reduced from about 38 nm to about 23 nm. However, more detailed polishing performance and the mechanism of chemical reactions during the polishing pr...

Preliminary Study on Fluidized Bed Chemical Mechanical Polishing (FB-CMP) Process for Stainless Steel 304 (SS304)

“…In their experiments, they showed that a high MRR can be obtained in stainless steel CMP when using a slurry of strong acidic conditions containing H 2 O. In addition to studies conducted by Hu et al, other studies reported a high MRR for acidic slurry [ 20 , 21 , 22 ]. Chen et al [ 23 ] used alumina slurry for stainless steel CMP, and attempted to determine the optimal slurry composition via a statistical approach.…”

“…Furthermore, Cheng et al [ 24 ] used alumina slurries and investigated the effects of pressure, rotation speed and particle size on material removal in stainless steel CMP. Jiang et al [ 21 ] conducted a study on stainless steel CMP using a slurry containing colloidal silica, H 2 O 2 , glycine and benzotriazole (BTA). In their studies, acidic slurries exhibited high MRR.…”

Perspective—Electrochemical Assessment of Slurry Formulations for Chemical Mechanical Planarization of Metals: Trends, Benefits and Challenges

“…In the FEOL, achieving an accurate polish level is another vital requirement for metal gate CMP. 2 Applications of metal CMP are not just limited to the field of microelectronics; they are routinely incorporated in the processing schemes of infrared detectors, 15 solar cell substrates, 16 flexible thinfilm transistors, 17 magnetic storage, 18 printed circuit boards 19 and nuclear detectors. 20 The present article, however, strictly focuses on the systems relevant for IC fabrications.…”

Chemical-Mechanical Impact of Nanoparticles and pH Effect of the Slurry on the CMP of the Selective Layer Surfaces

“…Since CMP MRR is dependent on the nanoparticle size and concentration in the slurry, the effects have been investigated by many researchers [26][27][28][29][30][31][32][33][34][35][36]. The reported results are often contradictory and are explained based on the selected ranges of the nanoparticle size and concentration, as well as by their behavior during the CMP process.…”

“…In the literature, there are numerous studies on similar topics [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] applied to the copper CMP of integrated circuits. It is worth mentioning that the present study intends to extend to other fields, namely to surfaces coated through selective transfer (the surfaces of the selective layer) in the friction process, considering the slurry pH (important for removal through CMP of the selective layer) [7,[22][23][24][25][26], H 2 O 2 (the most common oxidant) [27][28][29][30][31][32][33], size [21,[28][29][30][31], and concentration [12,21,34] of nanoparticles used in the CMP slurry. Thus, this paper explores the pH effect at a constant H 2 O 2 concentration on the etching and polishing behavior of the selective layer and the influences on size and concentration of nanoparticles during selective layer surface CMP.…”