Applied Science and Convergence Technology 2019; 28(5): 142-147
Published online September 30, 2019
Copyright © The Korean Vacuum Society.
You Jin Jia, Ki Seok Kima, Ki Hyun Kima, Ji Young Byuna, and Geun Young Yeoma,b,*
aSchool of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
bSKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University, Suwon 16419, Republic of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-CommercialLicense (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.
Silicon nitride (SiNx) thin films have attracted interest as an important material for use in next-generation devices such as a gate spacer in 3D fin field-effect transistors (finFETs), charge trap layers, etc. Many studies using the SiNx plasma enhanced atomic layer deposition (PEALD) method have been conducted, owing to its advantages over other SiNx deposition methods. In this review, the recent studies on PEALD of SiNx thin films are summarized, and the effects of some process parameters including plasma power, frequency, and process temperature on the material properties of SiNx are discussed. In addition, some properties of SiNx thin films such as conformality, wet etch rate, and others are reviewed.
Keywords: Silicon nitride (Si3N4), Plasma enhanced atomic layer deposition (PEALD), Process temperature, Step coverage, Wet etch rate
Recently, silicon nitride (SiNx) has attracted considerable interest owing to its diverse range of applications [1–10]. For instance, SiNx is used as a permeation barrier for flexible organic light emitting devices [7–10] or as a charge trap layer for logic and memory devices ; SiNx gate spacers have also been studied extensively [1–3,6]. High quality and excellent conformality are critical requirements for various applications of SiNx thin films. In addition, lowering the deposition temperature is an important factor for devices employing low temperature substrates such as polymer substrates. To satisfy such a requirement for employing SiNx thin films, many studies have used various deposition techniques such as chemical vapor deposition (CVD), low pressure chemical vapor deposition (LPCVD), plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), plasma enhanced atomic layer deposition (PEALD), and so on [11–43].
Low pressure chemical vapor deposition is the most common technique used to fabricate SiNx thin films because of its simple method and low cost. However, it is difficult to achieve a conformal layer on a high aspect ratio structure. In addition, the deposition needs to be conducted at high temperature (> 700 °C) [17,18]. Plasma enhanced chemical vapor deposition can deposit films at temperatures lower than that by using thermal LPCVD. Unfortunately, this leads to poor step coverage and low film quality [3,15,16]. To address issues related to conformality, the ALD technique has been studied extensively [19–26]; ALD is a cyclic process that offers atomic scale thickness control of the material that is being deposited. In addition, ALD methods can deposit thin films with high quality in terms of low wet etch rate and high conformality at low process temperatures [25,26]. However, ALD methods have several challenges such as a relatively high thermal budget for actual device application and low throughput (GPC > 2 Å/min) that hinders the industrialization of ALD methods [22,26]. By assisting with a plasma for dissociating reactive gases during ALD with PEALD processes, a thin film with a good step coverage on a high aspect ratio structure can be deposited at low temperatures. A plasma with reactant molecules can be used instead of exposure to reactant molecules only during the reactant exposure step; highly reactive species are formed during the reactant exposure step, which allows the deposition of high quality films with a high growth rate while lowering deposition temperatures [15,27–42].
In this paper, we briefly reviewed the recent work of PEALD SiNx and related process parameters that could determine film characteristics. Furthermore, this review will discuss properties of the film that are dependent on deposition conditions. A schematic of a PEALD cycle is shown in Fig. 1. Each SiNx PEALD cycle can be divided into four steps. In the first precursor adsorption step, similar to ALD, a Si precursor is introduced into the deposition chamber. Si precursors are chemisorbed on the surface through self-limiting reactions followed by a purge step. In the following plasma exposure step (for ALD, reactant exposure step), plasma-generated reactive species react with the adsorbed precursor on the surface. As a plasma source, capacitively coupled plasma (CCP) or inductively coupled plasma (ICP) sources are commonly used along with N2, NH3, or N2/H2 to generate reactive plasmas [5,15,27–40]. To optimize PEALD processes, various parameters should be adjusted to meet the material properties of SiNx required for the application.
As mentioned above, in PEALD, there are various process parameters including reactant gas, plasma source, precursor, precursor dose time, purge time, process temperature, and others. In this section, the effects of some process parameters influencing SiNx deposition are briefly discussed.
Many kinds of precursors such as trisilylamine (TSA), diisopropylaminosilane (DIPAS), bis(tertiary-butyl-amino)silane (BTBAS), trisdimethylaminosilane (3DMAS), di(
Plasma characteristics such as density of radicals, energy, and density of electrons and ions have a considerable influence on SiNx PEALD processes. Therefore, controlling plasma conditions such as rf power and frequency is important.
Another important parameter influencing plasma properties such as electron density and electron temperature is the frequency of plasma generation. Thus, the plasma generation frequency also needs to be controlled to improve SiNx thin film quality. King
PEALD methods can lower the deposition temperature compared to other deposition techniques. However, a considerably lower process temperature is still required for SiNx PEALD for various applications of SiNx films. For example, in the case of polymer or flexible substrates, very low process temperatures are required. The effect of the substrate temperature on the surface reaction mechanism in SiNx PEALD systems has been studied. Results showed that the PEALD process could lower the process temperature compared to other deposition techniques (LPCVD and ALD, etc.). However, the quality of SiNx thin films is still temperature dependent.
High defect density is the common reason for hysteresis in a capacitance–voltage (C–V) curve. Therefore, as shown in Fig. 7, the SiNx thin film deposited at 250 °C showed a considerably lower hysteresis curve. Although lower hysteresis was observed at lower PEALD temperature because of the high hydrogen content, crystallographic defects were found to be higher at lower deposition temperatures. In addition, hydrogen could be removed during processes such as annealing. Therefore, the deposition of SiNx at the lower PEALD temperature tends to cause more defect formation in the film.
In this section, we discuss common properties of SiNx thin films. These properties are important for applications of the film.
When the feature size of a semiconductor device decreases, high conformality on high aspect ratio structures is a critical requirement. Studies on SiNx PEALD focusing on the conformality of the film have been reported. For example, Faraz
The wet etch rate is highly correlated with the integrity and density of the film. Kim
In another study, Knoops
Besides studies on the wet etch rate of SiNx thin films on planar surfaces, the wet etch rate has also been studied for high aspect ratio structures. However, most studies showed different wet etch rates for the sidewall, bottom, and top surface of 3D trench patterns . The poor wet etch rate at the bottom sidewall is problematic in silicon nitride PEALD processes.
This brief review examined not only the deposition of SiNx thin films using the PEALD process, but also summarized the characteristics of SiNx thin films. Compared to other deposition methods, the PEALD process provides atomic scale thickness control and excellent conformality with a lower process temperature. Various SiNx PEALD processes have been studied, and some results have shown excellent film properties such as good step coverage and low wet etch rate. However, for device applications, several issues need to be solved. To address these issues, the effect of each PEALD process parameter (temperature, plasma power, exposure time, and gas composition) on the film growth mechanism needs to be understood in detail. Further, new plasma sources and precursors that can deposit highly conformal and dense thin films at low process temperatures for a variety of state-of-the-art devices, which require SiNx thin films, must be developed. Further studies on the deposition mechanism are required to enhance the quality of the deposited SiNx thin films.
Summary of recent studies on PEALD SiNx.
|Si precursor||Reactant||Plasma source/Frequency||Process temperature||GPC (Å/cycle)||Ref.|
|Trisilyamine (TSA, Si3H9N)||NH3||ICP
|Hexachlorodisilane (HCDS, Si2Cl6)||NH3||CCP
||350–450 °C||1.2 (400 °C)|
||400–500 °C||0.15 (500 °C)|
|Trisdimethylaminosilane (3DMAS, C6H19N3Si)||N2||ICP
|Dichlorosilane (DCS, SiH2Cl2)||NH3||0.24|
|Trysilylamine (TSA, Si3H9N)||N2||-|
|Bis(dimethylaminomethylsilyl)trimethylsilyl amine (DTDN2-H2, C9H29N3Si3)||N2||27.12 MHz||250–400 °C||0.36|
|Neopentasilane (NPS, (SiH3)4Si)||N2||CCP
|Trisilylamine (TSA, (SiH3)3N)||1.2|
||100–500 °C||~0.19 (100 °C)|
|Diisopropylaminosilane (DIPAS, C6H17NSi)||N2||CCP
||150–250 °C||0.3 (100 °C)|
|Pentachlorodisilane (PCDS, HSi2Cl5)||N2||Hollow cathode
|N2/NH3||~1 (270 °C)|
|Hexachlorodisilane (HCDS, Si2Cl6)||N2/NH3||Hollow cathode||270–360 °C||~0.68|
|1,3-di-isopropylamino-2,4-dimethylcyclosilazane (CSN-2, C8H22N2Si2)||N2||27.12 MHz||200–500 °C||0.43|
|NH3/N2 + N2||~0.35|
|Bis(tertiary-butyl-amino)silane (BTBAS, SiH2(NHtBu)2)||N2||ICP
|Bis(tertiary-butyl-amino)silane (BTBAS, SiH2(NHtBu)2)||N2||ICP
|Dichlorosilane (DCS, SiH2Cl2)||NH3||13.56 MHz||400–630 °C||1.39 (550 °C)|
Hydrogen concentration and film density of SiNx for different plasma power. Reproduced with permission from [
|Power (W)||H (at. %)||Density (g/cc)|