Atomic Layer Etching of HfO₂Using Sequential, Self-Limiting Thermal Reactions with Sn(acac)₂and HF

Abstract

The atomic layer etching (ALEt) of HfO 2 was performed using sequential, self-limiting thermal reactions with tin(II) acetylacetonate (Sn(acac) 2 ) and HF as the reactants. The HF source was a HF-pyridine solution. The etching of HfO 2 was linear with atomic level control versus number of Sn(acac) 2 and HF reaction cycles. The HfO 2 ALEt was measured at temperatures from 150-250 • C. Quartz crystal microbalance (QCM) measurements determined that the mass change per cycle (MCPC) increased with temperature from −6.7 ng/(cm 2 cycle) at 150 • C to −11.2 ng/(cm 2 cycle) at 250 • C. These MCPC values correspond to etch rates from 0.070 Å/cycle at 150 • C to 0.117 Å/cycle at 250 • C. X-ray reflectivity analysis confirmed the linear removal of HfO 2 and measured an HfO 2 ALEt etch rate of 0.11 Å/cycle at 200 • C. Fourier transform infrared (FTIR) spectroscopy measurements also observed HfO 2 ALEt using the infrared absorbance of the Hf-O stretching vibration. FTIR analysis also revealed absorbance features consistent with HfF 4 or HfF x surface species as a reaction intermediate. The HfO 2 etching is believed to follow the reaction: HfO 2 + 4Sn(acac) 2 + 4HF → Hf(acac) 4 + 4SnF(acac) + 2H 2 O. In the proposed reaction mechanism, Sn(acac) 2 donates acac to the substrate to produce Hf(acac) 4 . HF allows SnF(acac) and H 2 O to leave as reaction products. The thermal ALEt of many other metal oxides, as well as metal nitrides, phosphides, sulfides and arsenides, should be possible by a similar mechanism. Atomic layer etching (ALEt) is a thin film removal technique based on sequential, self-limiting surface reactions.1-3 ALEt can be viewed as the reverse of atomic layer deposition (ALD). 4 ALEt is able to remove thin films with atomic layer control. ALD and ALEt are able to provide the necessary processing techniques for surface engineering at the atomic level. 5,6 This atomic level control is needed for the nanofabrication of a wide range of nanoscale devices. 7Until recently, ALEt processes have been reported using only ionenhanced or energetic noble gas atom-enhanced surface reactions. [1][2][3] In these ALEt processes, a halogen is adsorbed on the surface of the material. Subsequently, ion or noble gas atom bombardment is used to desorb halogen compounds that etch the material. Using this approach, ALEt has been reported for Si,2,3,[8][9][10][11][12] Ge, 6,13 and compound semiconductors.14-17 ALEt has also been demonstrated for a variety of metal oxides. 7,[18][19][20] Additional ALEt studies have been conducted on various carbon substrates. 21-23The ALEt of Al 2 O 3 was recently reported using sequential, selflimiting thermal reactions with Sn(acac) 2 and HF as the reactants. 24The linear removal of Al 2 O 3 was observed at temperatures from 150-250• C without the use of ion or noble gas atom bombardment. Al 2 O 3 ALEt etch rates varied with temperature from 0.14 Å/cycle at 150• C to 0.61 Å/cycle at 250• C. 24 The Sn(acac) 2 and HF thermal reactions were both self-limiting versus reactant exposure. In addition, the Al 2...

Atomic Layer Deposition of AlF3 Using Trimethylaluminum and Hydrogen Fluoride

“…36,37 However, in this case, the etching of AlF 3 by TMA is not self-limiting. The AlF 3 etching at 250−300°C is dependent on the length of the TMA exposure.…”

Competition between Al2O3 atomic layer etching and AlF3 atomic layer deposition using sequential exposures of trimethylaluminum and hydrogen fluoride

“…4 In contrast, thermal processes have been developed only recently for ALE. [5][6][7][8][9][10] The recently developed thermal ALE processes are based on sequential fluorination and ligand-exchange reactions. 5,9,11 Fluorination converts the metal compound, such as a metal oxide, to a metal fluoride.…”

“…5,9,11 HF has been a successful fluorine precursor for Al 2 O 3 , HfO 2 , AlF 3 , and AlN ALE. [5][6][7][8][9][10] HF is also an effective fluorine precursor for the ALD of a variety of metal fluoridess such as AlF 3 , ZrF 4 , HfF 4 , MnF 2 , MgF 2 , ZnF 2 , and LiF. 12 Trimethylaluminum (TMA) is the most common precursor for Al 2 O 3 ALD and AlF 3 ALD.…”

Review Article: Plasma–surface interactions at the atomic scale for patterning metals

“…54 Since first formalized for Si etch using Cl 2 /Ar chemistry, 8,9 ALE has been shown to be viable in patterning SiO 2 , 7 Si 3 N 4 , 55-58 as well as other semiconductors and metal oxides. 59,60 While all these reported results show some level of efficacy in etching targeted materials, there is currently limited literature demonstrating ALE of metals. This is partly because metallic bonding, caused by the sharing of free electrons across the metal lattice, makes it challenging to direct the bond formation between metals and the gasphase chemistries to realize self-limiting reactions that are essential to ALE.…”