HF/H[sub 2]O[sub 2] Etching for Removal of Damage Layer on As-Transferred Si Layer Formed by Ion-Cut Process

Abstract

When a 100 nm thick Si layer was transferred onto a bare Si wafer by the hydrogen-induced-layer-transfer process, a spongy damage layer with microvoids was formed on the transferred layer because of hydrogen blistering. The surface-to-volume ratio of the damage layer was greater than that of the layer where blistering did not occur. Therefore, the damage layer was selectively etched by an HF/H 2 O 2 mixture and completely removed ͑the etching rates of Si at 70°C for the bulk layer and damage layer were 0.45 and 13.8 nm/min, respectively͒. Consequently, a smooth, damage-free Si layer ͑root-mean-square roughness = 2.74 nm͒ was obtained.The hydrogen-induced-layer-transfer process, also known as the ion-cut process or smart cut process, is employed to fabricate silicon-on-insulator ͑SOI͒ wafers used in the manufacture of highend integrated circuits. 1 In this process, H + ions ͑dose: 3.5 ϫ 10 16 -1.0 ϫ 10 17 cm −2 2 ͒ are implanted in a Si wafer capped with a dielectric layer to obtain a layer of H + ions at a certain depth ͑hereafter referred to as "buried layer"͒ forming the desired layer for layer transfer. Subsequently, the H + -implanted wafer and a handle wafer are bonded together by an appropriate wafer bonding technique.Annealing the bonded pair at 500-600°C results in the formation of hydrogen microbubbles, and the aggregation of these microbubbles in the buried layer leads to crack propagation around the damage peak formed during ion implantation. 2 The desired layer is then transferred to the surface of the handle wafer. The surface of the as-split layer is rough because the buried layer is fractured by hydrogen blistering; the root-mean-square ͑rms͒ roughness of the as-split layer is approximately 8-11 nm 3 at an implant dose of 10 17 cm −2 of H + . Most of the defects generated during H + ion implantation are distributed on one side of the transferred layer, which results in the formation of a spongy damage layer on the transferred layer after the annealing process. The cross-sectional transmission electron microscopy ͑TEM͒ image in Fig. 1 shows that most of the lattice damage layer is filled with hydrogen ions, which is introduced by the implantation ͑H 2 + ion dose: 4 ϫ 10 16 cm −2 , 160 keV͒ on one side of the transferred layer. The as-implanted silicon wafer is covered by a 300 nm thick polycrystalline silicon layer and a 100 nm thick SiO 2 layer.The device performance of a fully depleted ͑FD͒ transistor is strongly dependent on the thickness of the SOI layer. In FD nanodevices, the thickness of the Si films should be controlled within an accuracy of a few nanometers for a good parameter reproducibility. For example, to suppress short-channel effects ͑SCEs͒ in 70 nm FD SOI transistors, it is necessary to reduce the thickness of the SOI layer to within 30 Ϯ 5% nm. 4 However, it is difficult to maintain a uniform thickness when the well-defined layer is thinned by chemical mechanical polishing ͑CMP͒.Because of the low mass of H + ions, it is usually necessary to thin the transferred layer by CMP for ac...

Etching Silicon with HF-H2O2-Based Mixtures: Reactivity Studies and Surface Investigations

“…The moderate etching rates are accompanied by good cleaning characteristics and a selective etching of SiO 2 layers compared to Si . This enables applications in Si-wafer cleaning (e.g., wafer surface analysis (WSA), purification of poly-Si chunks) and fabrication of silicon-on-insulator (SOI) devices …”

“…25 This enables applications in Si-wafer cleaning (e.g., wafer surface analysis (WSA), 26 purification of poly-Si chunks 3 ) and fabrication of silicon-on-insulator (SOI) devices. 27 Etching of silicon in HF-H 2 O 2 mixtures was described as a two-step reaction with the intermediary formation of SiO 2 , which is dissolved by HF to form H 2 SiF 6 (eqs 7−9). 27 A critical discussion of this simple reaction scheme is necessary because it is well-known that for HF-containing etching mixtures normally Si−H groups and no SiO 2 or SiO x are formed on the surface, as unambiguously shown for HF-HNO 3 mixtures by Zanoni et al in 1998.…”

“…27 Etching of silicon in HF-H 2 O 2 mixtures was described as a two-step reaction with the intermediary formation of SiO 2 , which is dissolved by HF to form H 2 SiF 6 (eqs 7−9). 27 A critical discussion of this simple reaction scheme is necessary because it is well-known that for HF-containing etching mixtures normally Si−H groups and no SiO 2 or SiO x are formed on the surface, as unambiguously shown for HF-HNO 3 mixtures by Zanoni et al in 1998. 28 It is proposed in the literature 29 that there is no typical chemical oxidation of Si (surface atoms).…”

HF-(NH4)2S2O8-HCl Mixtures for HNO3- and NOx-free Etching of Diamond Wire- and SiC-Slurry-Sawn Silicon Wafers: Reactivity Studies, Surface Chemistry, and Unexpected Pyramidal Surface Morphologies

“…Etching of silicon in hydrofluoric acid-based mixtures is basically described as a two-step process. The oxidation of silicon is followed by dissolution of the oxidized silicon species by fluoride containing species in order to form SiF 6 2– . ,− The oxidation of silicon surface atoms in hydrofluoric acid-containing mixtures requires a standard redox potential of +0.7 V . Therefore, nitric acid, hydrogen peroxide, chromium­(+6) oxide, and numerous other species conduce as oxidants. ,, For industrial etching processes, nitric acid is the most common oxidant.…”

Transforming a Ion-Cut Silicon Layer From Damage-Dense to Defect-Free Status Via Microwave-Anneal and Recrystallization

“…Therefore to achieve a defect-free layer, the as-split thickness needs to include an allowance space adjoining to the cavity-filled dense damage layer for additional thickness thinning through polishing or etching to remove the defects [3].…”