Precise Depth Control and Low-Damage Atomic-Layer Etching of HfO[sub 2] using BCl[sub 3] and Ar Neutral Beam

Abstract

The etch characteristics of HfO 2 by atomic-layer etching ͑ALET͒ were investigated using a BCl 3 /Ar neutral beam. The effect of ALET on surface modification and etch-depth control was also examined. Self-limited etching of HfO 2 could be obtained using BCl 3 ALET. This was attributed to the absorption of BCl 3 by the Langmuir isotherm during the absorption stage and the vaporization of hafnium-chlorides/boron oxychlorides formed on the surface during the desorption stage. In addition, the surface composition of HfO 2 was not altered by etching during ALET.The downscaling of metal-oxide-semiconductor field-effect transistors ͑MOSFETs͒ has created the need for high-dielectric-constant ͑k͒ materials to replace SiO 2 for reducing the gate-leakage current while maintaining the gate-dielectric capacitance. Among the various high-k materials, hafnium oxide ͑HfO 2 ͒ is a potential material in next-generation MOSFETs due to its high-dielectric constant ͑25-30͒, wide bandgap, low leakage current, and superior thermal stability. 1,2 In order to apply HfO 2 to next-generation MOSFET devices, HfO 2 films should be patterned by dry etching, because the wet etching of HfO 2 has several disadvantages such as difficulty in controlling the etch rate, severe undercut, etc. Therefore, there have been many studies on the dry etching of HfO 2 using halogen-based plasma etching for applications to MOSFET devices. A precise etch rate is required in the plasma etching for HfO 2 instead of a high etch rate due to the low thickness of the material. Moreover, an extremely high etch selectivity over the underlayer material is required. In addition, the damage on the etched surface is intolerable. However, these conventional plasma-etching processes tend to physically damage the surface of the devices by creating surface defects, including structural disruption, an intermixing layer, or stoichiometry modification, and increasing surface roughness, due to use of energetic reactive ions to achieve vertical etch profiles. 3,4 In addition, these halogen-based plasma etchings showed finite etch selectivity between HfO 2 and the underlying materials. 5,6 These problems decrease the device performance.Atomic-layer etching ͑ALET͒ may be the most suitable method for etching HfO 2 in next-generation MOSFET devices because it may etch HfO 2 with no physical damage and with atomic-scale etch controllability. Generally, ALET is a cyclic process that consists of four sequential steps: ͑i͒ adsorption of reactant molecules on the surface, ͑ii͒ evacuation of the remaining reactant molecules, ͑iii͒ Ar + ion-beam irradiation to the reactant adsorbed surface for the desorption of chemisorbed species, and ͑iv͒ evacuation of the etch products. 7,8 However, during step ͑iii͒ with Ar + beam irradiation, the device can be damaged electrically due to the charged particles, such as positive ions generated in the plasma.In this article, ALET of HfO 2 was carried out using BCl 3 adsorption, followed by Ar neutral-beam irradiation instead of Ar + ionbeam irradiation to a...

Overview of atomic layer etching in the semiconductor industry

“…These tests can help indicate to what degree the ALE process approaches ideal behavior, albeit nonideal conditions may still yield significant benefit. In addition to these tests, ultimately, ALE will be characterized by the benefits on the wafer: features with atomically smooth and flat surfaces, 6,49,50 reproducibility of stoichiometry, [50][51][52][53][54] uniformity across the wafer, 6 etching without loading (pattern-density independence), 55 low-damage to remaining material, 28,50,[56][57][58] and etch selectivity to other materials. 44,57,[59][60][61] The first test is the synergy test, which measures the amount of material etched per cycle (typically averaged over many cycles) compared to the amount from each individual step.…”

“…We will discuss delivery of the chlorine thermally as Cl 2 gas and also with plasma to increase the reaction rate by dissociating Cl 2 into reactive species. Chlorine-based chemistry is commonly used in ALE for silicon, as well as germanium, 62 metal oxides, 51,52,63 and III-V materials. 40,[64][65][66][67][68][69] After purging the excess chlorine from the chamber, an Ar plasma provides ion bombardment to activate removal of the silicon chloride reactive layer (reaction B), followed by purging of by-products.…”

“…These include electron cyclotron resonance plasma, 33,39,56,62,86,87 helicon source plasma, 49,88 TCP, 6 capacitively coupled plasma, 5,60,89 and inductively coupled plasma. 90 Other types of energetic species have also been evaluated, including ion beams, 40,48 neutral beams, 35,[50][51][52]68,69,83,[91][92][93][94][95][96] and laser beams. 32,66,67,97,98 While specialized equipment certainly plays an integral part in basic understanding of ALE, it may be more practical to adapt conventional etching equipment for use in ALE.…”

Atomic Layer Etching at the Tipping Point: An Overview

“…For instance, self-limited etching of HfO 2 at 1 ML/cycle using BCl 3 and energetic Ar neutral beam bombardment required a BCl 3 pressure above a critical pressure of 0.22 mTorr for 20 s BCl 3 exposure and a critical dose for energetic particle bombardment of 1.48×10 17 atoms/cm 2 , respectively. 47 Corresponding information has been published for other reactant/materials systems.…”

Atomic Layer Etching: What Can We Learn from Atomic Layer Deposition?

“…They showed a considerable improvement in the electrical characteristics of the devices prepared by ALEt when compared to results obtained for devices etched by conventional wet or dry etch techniques. 38,39 These improved results were attributed to a low edge damage of the gate oxide during ALEt by maintaining the surface composition at the edge of the gate oxide together with exact control of the Si etch depth. Recently, Metzler et al 40 also demonstrated ALEt of SiO 2 using a steady-state Ar plasma, periodic injection of a defined amount of C 4 F 8 gas and synchronized plasma-based Ar + ion bombardment.…”