Influence of the Secondary Ion Beam Source on the Laser Damage Mechanism and Stress Evolution of IBS Hafnia Layers

Round 1

Reviewer 1 Report

Dear authors,

The paper focuses on the evolution of stress and defects, which is one of major concerns in optical coating applications, and successfully investigate the effect of assist ion beam on phase, crystallinity, and residual stress. As I read the whole paper, this paper can be accepted with minor corrections. Here are a few suggestions that may improve the quality of the paper.

[Suggestions]

1. As far as I can see, the deposition temperature is fixed around 100 degC. However, from my past experience, thin film deposition and additional ion irradiation from the assist ion source would increase the surface temperature. So I would recommend to add an explanation regarding possible temperature increase caused by assisting ion beam. That helps readers understand the situation of the deposition more clearly.
2. Stress evolution as a function of ion bombardment energy has been modeled by several research groups. I recommend you to visit these references to discuss your results on stress evolution as a function of U_assist. I hope it helps improve the discussion.
   1. M. M. M. Bilek and D. R. McKenzie, Surf. Coat. Technol. 200 (14 15), 4345 (2006)
   2. C. A. Davis, Thin Solid Films 229 , 30 (1993)

1. In the method section, the gas condition is described with flow rates, but it would be nice if you can add the pressure value so that the readers can know the more general idea of deposition condition. i.e. Even the same flow rate may result in different situations when pumping speed is different. Chamber pressure is a more general parameter for thin film deposition.

[Minor corrections]

1. L49 "2.1. HfO2 thin film deposition": "2" should be in subscript
2. L132 Fig. 2a and 2b: The units on x-axis are missing. "Assist source voltage (V)" and "2Θ (degrees)"

Author Response

Dear Reviewer

thank you for the interesting comments. In the following you can find my detailed replies.

"As far as I can see, the deposition temperature is fixed around 100 degC. However, from my past experience, thin film deposition and additional ion irradiation from the assist ion source would increase the surface temperature. So I would recommend to add an explanation regarding possible temperature increase caused by assisting ion beam. That helps readers understand the situation of the deposition more clearly."

Indeed it is the case, we have some preliminary information about the maximum temperature achieved during the sputtering deposition. Investigating the effect of temperature in a systematic fashion is very high in our priority list, for the sake of clarity, however we decided to skip those preliminary results from this manuscript.

We added the following line to the text (page 2 line 65): "Since the DIBS system is not equipped with an active temperature stabilization, the temperature can increase during the operation of the assisting source."

"Stress evolution as a function of ion bombardment energy has been modeled by several research groups. I recommend you to visit these references to discuss your results on stress evolution as a function of U_assist. I hope it helps improve the discussion.

1. M. M. M. Bilek and D. R. McKenzie, Surf. Coat. Technol. 200 (14 15), 4345 (2006)
2. C. A. Davis, Thin Solid Films 229 , 30 (1993)"

Thank you for the literatur suggestion. We cited the references and added the following line to the text (page 7, line 193):

"The macroscopic cracking / delamination and eventual blistering can explain why we observed a sharp decline in the net stress, which is not expected according to the Davis or Bilek-McKenzie models [9, 10]."

"In the method section, the gas condition is described with flow rates, but it would be nice if you can add the pressure value so that the readers can know the more general idea of deposition condition. i.e. Even the same flow rate may result in different situations when pumping speed is different. Chamber pressure is a more general parameter for thin film deposition."

We added the chamber pressure (3.5±0.5×10^-3 torr ) to the text.

On the other hand our setup has 6 operating gas inlets (Neutralizer1: Ar; Neutralizer2: Ar; Primary source: Ar; Assisting source: Ar & O₂; Target flooding: O₂) therefore the mixture used by the assisting source is the more decriptive one, at least for systems with similar geometry.

Reviewer 2 Report

In this work, the effects of the secondary ion beam source as well as the ion beam energy on various properties of HfO2 films deposited by IBS, such as microstructure, surface morphology, stress, and optical properties, were experimentally investigated. It is found that energetic ion bombardment increases the compressive stress of the deposited films, leading to densification, crystallization, and even cracking. The manuscript is well-written and well-organized. I recommend publishing after a minor revision. My comments are as follows.

The effect of energetic ion bombardment on thin film deposition is an interesting and important research topic. Readers familiar with this research field may recall first the structure zone diagram (SZD) proposed by Anders (Anders A., A structure zone diagram including plasma-based deposition and ion etching, Thin Solid Films 518(15), 4087-4090, 2010) that includes the effects of energetic ions. That is, the deposition rate and the properties of deposited thin films such as the microstructure can be approximately determined by a generalized homologous temperature and a normalized kinetic energy flux. The results presented in this manuscript seem to be still well-described by the SZD. Therefore, it may be very helpful to know in which zone or zones of the SZD (zone 1, zone T, zone 2, or zone 3?) the films in this study belong to, for a more profound understanding of the range of parameters studied in the manuscript.

Also, there is a typo in the abstract that in line 12, "(LIDT)" should come after "threshold".

Author Response

Dear Reviewer,

thank you for your interesting comments. Below are my detailed answers.

"The effect of energetic ion bombardment on thin film deposition is an interesting and important research topic. Readers familiar with this research field may recall first the structure zone diagram (SZD) proposed by Anders (Anders A., A structure zone diagram including plasma-based deposition and ion etching, Thin Solid Films 518(15), 4087-4090, 2010) that includes the effects of energetic ions. That is, the deposition rate and the properties of deposited thin films such as the microstructure can be approximately determined by a generalized homologous temperature and a normalized kinetic energy flux. The results presented in this manuscript seem to be still well-described by the SZD. Therefore, it may be very helpful to know in which zone or zones of the SZD (zone 1, zone T, zone 2, or zone 3?) the films in this study belong to, for a more profound understanding of the range of parameters studied in the manuscript."

We cited the mentioned reference and extended our discussion with the following:

"The deposition of thin films can be characterized by the structure zone diagram (SZD), which takes the generalized deposition temperature, the ion energy as well as the film thickness into account [22]. In case of U_assist=0 only the primary sputtered molecules from the target reach the specimen surface. These are characterized by a few tens of eV energy at most. The substrate temperature is most probably close to the set 100 °C (~0.1 T_m for HfO₂) thus the deposition take place at the border of Zone1 and ZoneT. The mean free path for oxygen molecules and argon atoms for an operating pressure of 3.5×10^-3 torr is larger than the assisting source-substrate distance thus most of the species reach the specimen without any energy loss. Accordingly a deposition at the border of ZoneT and Zone3 takes place. This is in agreement with the observed crystallization of the layers."