Suppression of Acoustic Resonances in BST-Based Bulk-Ceramic Varactors by Addition of Magnesium Borate

Round 1

Reviewer 1 Report

The paper shows good results on suppression of acoustic resonances in BST based bulk ceramics applied for tunable capacitors through the addition of a second phase of Mg3B2O6. The tunability is decreased by a factor of 3, though. However, this loss is compensated by an increase of the quality factor by the same factor of 3. This improvement brings a lot in cases where the quality factor is more important than tuning range. In the conclusions it is stated that the change is due to a reduction (by 10 %) of the volume fraction high-K tuning material (BST). But the electric field is also decreasing in the BST phase. It would be illustrative to add also a table or figure with the zero-field dielectric constant as a function of secondary phase volume fraction.

The introduction is somewhat too simplified. Piezoelectric coefficient and electrostrictive coefficient are not independent, and also for the application of tunable capacitance there are 2 distinct electric fields, a larger tuning electric field, which is by moments constant, and an AC electric field of the signal passing through the capacitor. In addition, the notation is not standard. The electrostrictive coefficient is not abbreviated by “d” (which is a piezoelectric coefficient) but by Q. I think it would be nice to state in the introduction that the piezoelectric activity is automatically arising by a strong tuning electrical field. I wrote down in the attachment how one has to make the connection. The authors should improve the introduction.

Summary for minor, obligatory corrections:

• Improve introduction

Add dielectric constants (V=0) as a function of second phase (should be hidden in ESR).

Comments for author File: Comments.pdf

Author Response

The paper shows good results on suppression of acoustic resonances in BST based bulk ceramics applied for tunable capacitors through the addition of a second phase of Mg3B2O6. The tunability is decreased by a factor of 3, though. However, this loss is compensated by an increase of the quality factor by the same factor of 3. This improvement brings a lot in cases where the quality factor is more important than tuning range. In the conclusions it is stated that the change is due to a reduction (by 10 %) of the volume fraction high-K tuning material (BST). But the electric field is also decreasing in the BST phase. It would be illustrative to add also a table or figure with the zero-field dielectric constant as a function of secondary phase volume fraction.

• Thanks for your welcoming. The author will include this in the manuscript in the revised version.

The introduction is somewhat too simplified. Piezoelectric coefficient and electrostrictive coefficient are not independent, and also for the application of tunable capacitance there are 2 distinct electric fields, a larger tuning electric field, which is by moments constant, and an AC electric field of the signal passing through the capacitor. In addition, the notation is not standard. The electrostrictive coefficient is not abbreviated by “d” (which is a piezoelectric coefficient) but by Q. I think it would be nice to state in the introduction that the piezoelectric activity is automatically arising by a strong tuning electrical field. I wrote down in the attachment how one has to make the connection. The authors should improve the introduction.

• Thanks for your detailed explanation. The author acknowledges the missing clarity in the explanation of the concept of lowering acoustics with lowering permittivity here and will make few changes to the revised manuscript to get close to the explanation.

Reviewer 2 Report

This paper describes the realization and the characteziation of specific ferroelectric varactors. The authors demonstrate that the introduction of 10% MBO contribute to the drastic reduction of acoustic phenomena. The manuscript is well organized and the authors give a complete and rigourous study on the material properties and its impact on performances. After some modifications/corrections I recommend publication in the CRYSTALS journal.

The authors says « MEMS-based solutions suffer from limited power handling and are also prone to mechanical reliability issues especially under hot switching conditions, which are essential for plasma applications » The authors should moderate their comments because some solutions based on MEMS technology describe very good results in terms of reliability and power handling sufficiently relevant for commercial applications.

Solutions based on Phase Change Materials (PCM) exist and deserve to be addressed in the introduction.

Authors don’t sufficiently develop why high permittivity would be a problem. My feeling is that this isn’t the issue because the topology chosen can make it possible to overcome this high permittivity.

Does the fact that the electric field propagates in the entire thickness of the material (parallel plate configuration) not contribute to an increase in electrostritive phenomena? Doesn't a CPW type (coplanar) configuration where the field propagates parallel to the substrate reduce these effects ?

Some publications claim that there is a dead layer at the electrode/ material interface with specific acoustic velocity which would contribute to the electrostrictive phenomena. Can the authors address this aspect ?

In order to compare the results proposed by the authors with the state of the art, it would be interesting to specify the equivalent electric field applied rather than the bias voltage.

Although it is undeniable that MBO has an impact on the reduction of acoustic phenomena, it is noted that the tunability drops drastically which can call into question the interest of this type of component.

Author Response

Highly appreciate your time to review the paper. The author hopes that he has replied to the comments in a proper manner.

This paper describes the realization and the characteziation of specific ferroelectric varactors. The authors demonstrate that the introduction of 10% MBO contribute to the drastic reduction of acoustic phenomena. The manuscript is well organized and the authors give a complete and rigourous study on the material properties and its impact on performances. After some modifications/corrections I recommend publication in the CRYSTALS journal.

• Author would like to thank you for your welcoming comments.

The authors says « MEMS-based solutions suffer from limited power handling and are also prone to mechanical reliability issues especially under hot switching conditions, which are essential for plasma applications » The authors should moderate their comments because some solutions based on MEMS technology describe very good results in terms of reliability and power handling sufficiently relevant for commercial applications.

• The author would like to suggest the change of the tone of the line in the way- ‘MEMS-based solutions have been promising alternative but there is still scope of improvement in terms of power handling and mechanical reliability under hot switching conditions, which are essential for plasma applications.’

Solutions based on Phase Change Materials (PCM) exist and deserve to be addressed in the introduction.

• The author acknowledges the fact that these materials can produce notable tunable behavior but most of the literature around the work is present on the mm-wave frequencies or above which is out-of-scope of this work.

Authors don’t sufficiently develop why high permittivity would be a problem. My feeling is that this isn’t the issue because the topology chosen can make it possible to overcome this high permittivity.

• The author acknowledges the comment and would improve the theory here. It is suspected that the topology change will influence the power-handling and the tunability capabilities in a negative way.

Does the fact that the electric field propagates in the entire thickness of the material (parallel plate configuration) not contribute to an increase in electrostritive phenomena? Doesn't a CPW type (coplanar) configuration where the field propagates parallel to the substrate reduce these effects ?

• The author would like to acknowledge that the suggestion is interesting and would like to investigate more in the future. Moreover, in this work, the main focus is on the MIM approach as it is suspected that, this approach provides more tunability which is essential for the discussed application environment. Also, limited power handling and inductive behavior are other potential bottlenecks for the planar approach.

Some publications claim that there is a dead layer at the electrode/ material interface with specific acoustic velocity which would contribute to the electrostrictive phenomena. Cinclusion an the authors address this aspect?

• As the work discusses the bulk-ceramic approach, the author is neglecting the dead layer here as it is very thin in this technology. According to the author’s understanding, the dead layer phenomenon is mostly discussed with the thin-film applications where its effects are making an impact.

In order to compare the results proposed by the authors with the state of the art, it would be interesting to specify the equivalent electric field applied rather than the bias voltage.

• The author will include that in the characterization part.

Although it is undeniable that MBO has an impact on the reduction of acoustic phenomena, it is noted that the tunability drops drastically which can call into question the interest of this type of component.

• The author has motivated the work by taking the high-Q applications into consideration provided the required tunability is also achieved. The trade-off between Q and tunability is important for high-performance varactors. This is shown by approx. 3 times decrease in tunability and 3 times increase in quality factor. This point is also stated by the second reviewer.

Thanks and regards,

Prannoy Agrawal