A Study on Reactive Ion Etching of Barium Strontium 

Titanate Films Using Mixtures of Argon (Ar), 

Carbon Tetrafluoride (CF4), and Sulfur Hexafluoride (SF6)

by Samuel G. Hirsch, Ryan C. Toonen, Eric H. Ngo, Mathew P. Ivill, 

and M. W. Cole

ARL-TR-6979 July 2014

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Army Research Laboratory

Aberdeen Proving Ground, MD 21005-5069

ARL-TR-6979 July 2014

A Study on Reactive Ion Etching of Barium Strontium 

Titanate Films Using Mixtures of Argon (Ar),

Carbon Tetrafluoride (CF4), and Sulfur Hexafluoride (SF6)

Samuel G. Hirsch, Ryan C. Toonen, Eric H. Ngo, Mathew P. Ivill, 

and M. W. Cole

Weapons and Materials Research Directorate, ARL

Approved for public release; distribution is unlimited.
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A Study on Reactive Ion Etching of Barium Strontium Titanate Films Using 

Mixtures of Argon (Ar), Carbon Tetrafluoride (CF4), and Sulfur Hexafluoride 

(SF6)

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Samuel G. Hirsch, Ryan C. Toonen, Eric H. Ngo, Mathew P. Ivill, and M. W. Cole

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Aberdeen Proving Ground, MD 21005-5069

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14. ABSTRACT

Barium Strontium Titanate (BST) is a complex oxide material with ferroic properties which has been considered for 

applications ranging from non-volatile memory to microwave tunable devices. When grown in bulk films BST forms a 

continuum of domains. It is theorized however, that when the material can be grown on the order of a single domain, its 

properties will drastically change. To exploit the ferroic properties of BST we developed a device fabrication method utilizing 

self-aligned etching to create metal-insulator-metal (MIM) varactors. As part of this method we employed reactive ion etching 

(RIE) to remove BST and create cylindrical island stacks consisting of platinum top electrodes, atop a layer of BST, atop a 

platinum bottom electrode film, all on top of a sapphire substrate. Here we report and compare the results of a study on using 

RIE to remove BST using combinations of three gas, argon (Ar), carbon tetrafluoride (CF4), and sulfur hexafluroride (SF6).

15. SUBJECT TERMS

reactive ion etching, barium strontium titanate, argon, carbon tetrafluoride, sulfur hexafluoride

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19a. NAME OF RESPONSIBLE PERSON

Samuel G. Hirsch

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iii

Contents

List of Figures iv

1. Introduction and Background 1

2. Experimental Procedure 2

3. Results and Discussion 3

4. Conclusions 5

List of Symbols, Abbreviations, and Acronyms 6

Distribution List 7
iv

List of Figures

Figure 1. Amount of BST removed vs. etch time for Ar:CF4..........................................................3

Figure 2. Amount of BST removed vs. etch time for Ar:SF6. .........................................................4

Figure 3. SEM cross-section of varactor showing the Pt bottom electrode, BST film, and Pt 

TE...............................................................................................................................................5
1

1. Introduction and Background

Barium strontium titanate (BST) is a complex oxide material with ferroic properties that has been 

considered for applications ranging from nonvolatile memory to microwave tunable devices.1

Ferroic materials fall into four primary orders that include ferroelectricity, ferromagnetism, 

ferroelasticity, and ferrotoroidicity (which has not yet been observed). A material that exhibits 

two or more of these properties is known as a multiferroic. When grown in bulk or as thick films 

these materials form a continuum of individual domains. It is theorized, however, that if these 

materials can be structured with physical dimensions on the order of the size of a single domain, 

their ferroic properties will drastically change.2,3 For a collection of nanoparticles with radii 

smaller than a magnetic exchange length an effect known as superparamagnetism is predicted.

4

In order to exploit the properties of ferroic thin film materials these materials must be configured 

into device structures. One of the most critical post growth process science methods used to 

construct practical device structures is dry etching. Dry etching, namely reactive ion etching 

(RIE), of the ferroic thin film material enables the pattern delineation to create the device 

structure. A novel method to create micro/nano structures is the self-aligned RIE technique. This 

study investigates and develops the self-aligned RIE technique to create small-scale BST device 

structures. This method utilizes traditional photolithography techniques and RIE to create metalinsulator-metal (MIM) varactors. Typically, dry etching techniques use a radio frequency (RF)

power supply to place a high-frequency voltage difference between two parallel metal plates on a 

chamber held at low vacuum. Argon (Ar) gas is used to physically bombard the material to be 

etched. RIE introduces an additional gas or gases, that when broken down, generate reactive ions 

that also chemically attack the material to be etched. In this report, Ar is used in various 

concentrations with carbon tetrafluoride (CF4) and sulfur hexafluoride (SF6) to construct MIM 

varactors, made of Pt/BST/Pt* on sapphire substrates, using RIE.

 1Kumar, A.; Manavalan, S. G. Characterization of Barium Strontium Titanate Thin Films for Tunable Microwave and DRAM 

Applications. Surface & Coatings Tech 1–3 August 2005, 198, 406–413.

2Kittel, C. Phys. Rev. 1946, 70, 965–971.

3Toonen, R.; Cole, M. Third-Order Electric-Field-Induced Dipolar Resonances From Patterned Barium-Strontium-Titanate 

Thin-Films. App Phys Lett. 2012, 100, 222908.

4Rüdiger A.; Schneller, T.; Roelofs, A.; Tiedke, S.; Schmitz, T.; Waser, R. Nanosize Ferroelectric Oxides – Tracking Down 

the Superparaelectric Limit. Appl. Phys. A 2005, 80, 12471255.

*Pt is platinum.
2

2. Experimental Procedure

Whole, 2-inch-diameter, 330-μm-thick, (0001) epi-ready sapphire wafers were sputter cleaned in 

Ar using a Lesker CMS18 sputter unit. Subsequent to the substrate cleaning, two layers of film 

were deposited. First, a layer of Pt was direct current (DC) sputter deposited to create a bottom 

electrode. Then, the ferroic film, a layer of B60S40TiO3,

* was RF sputter deposited over the Pt 

bottom electrode. Following the film depositions the wafer was cleaved into 1 × 1-cm-square test 

structures, whereby several samples were randomly selected for scanning electron microscopy 

(SEM) analyses to determine Pt and BST layer thicknesses. Pt layers ranged from 120 to 225 nm 

while BST layers ranged from 165 to 330 nm. The films themselves were very uniform over any 

given wafer. Hence, the wide variations reported here are due to differences from wafer to wafer.

To construct the MIM varactors the following photolithography steps were employed. First, a lift 

off resist, LOR 30B (MicroChem, Inc.), was spin coated at 2000 revolutions per minute (rpm)

for 1 min. Next, samples were soft baked at 180 °C on a hot plate for 5 min. Then photoresist, 

SC-1827 (Shipley), was spin coated at 5500 rpm for 30 s followed by a hard bake at 95 °C for 

30 min. Next, samples were exposed to 405-nm ultraviolet (UV) light for 18 s at 10 mW/cm2†

using a mask-aligner in contact mode with a chrome mask. Samples were then developed in MF319 developer for 105 s followed by a 15 s bath in deionized (Di) water and then a 105 s bath in 

Di water. Samples were dried with nitrogen (N2) then placed into a sputter chamber for 

deposition of the Pt top electrode (TE) that will serve as the etch mask in the RIE process. The 

DC sputter deposition parameters were optimized to produce a TE thickness of approximately 

160 nm. Finally, samples were soaked in Remover PG (MicroChem) at 55 °C overnight with 

subsequent removal of underlying lift-off-resist (LOR) revealing islands of circular, Pt TEs on 

BST.

Reactive ion etching was carried out using a March CS-1701 Reactive Ion Etcher. To supply 

power to the RIE a RFX-600 RF generator was used to deliver 500 W at 13.56 MHz with an 

output impedance of 50 ohms. The gases that were used were combinations of Ar and CF4, and 

Ar and SF6. Vacuum pressures within the March RIE were 200 mTorr with a total gas flow rate 

of 20 sccm‡. Etching times ranged from 4 to 30 min. Once etched samples were then cleaved in 

half and analyzed using a SEM to determine subsequent thickness of BST.

 *Notation indicates that the percentage of B to S is 60:40. Also implied in this notation is that proportionally, B+S=T or 

(B+S):T.

† milliWatt per square centimeter.

‡Standard cubic centimeter per minute.
3

3. Results and Discussion

Samples were run in mixtures of Ar:CF4 and Ar:SF6 at concentrations of both 20:80 and 40:60. 

Results of these mixtures are plotted in figures 1 and 2 that compare etchant concentrations.

Averaging the data yielded etching rates of 5.7 and 6.1 nm/min for Ar:CF4 20:80 and 40:60, 

respectively, and 7.5 and 8.0 nm/min for Ar:SF6 20:80 and 40:60, respectively. Immediately it 

shows that increasing the argon to chemical etchant ratio increases the etch rate slightly while 

switching from CF4 to SF6 increases the etch rate more significantly. In fact an initial etch of 

180 nm of BST for 30 min with Ar:SF6 at 20:80 found the film to be completely etched away. 

Hence, shorter etch times were necessary for Ar:SF6.

Figure 1. Amount of BST removed vs. etch time for Ar:CF4.
4

Figure 2. Amount of BST removed vs. etch time for Ar:SF6.
5

Upon visual inspection most samples appeared to indicate that the etched films were very 

uniform as displayed by their evenness of color over their entire surface. Figure 3 is a SEM 

micrograph cross-section of a typical MIM varactor showing BST film sandwiched between two 

Pt electrode layers, is a good representation of this uniformity.

Figure 3. SEM cross-section of varactor showing the Pt bottom electrode, BST film, and Pt TE.

4. Conclusions

In an effort to develop a process science method to construct self-aligned MIM varactors, layers 

of Pt followed by a ferroic film, B60S40TiO3, were first deposited on (0001) epi-ready sapphire 

substrates. The 1 × 1-cm samples were then cleaved and traditional photolithography was

subsequently used to deposit circular, Pt TEs onto the BST. A dry etch process via RIE was then 

conducted using combinations of Ar:CF4 and Ar:SF6 at ratios of both 20:80 and 40:60. Etching 

rates were found to be 5.7 and 6.1 nm/min for Ar:CF4 20:80 and 40:60, respectively, and 7.5 and 

8.0 nm/min for Ar:SF6 20:80 and 40:60, respectively. Etched samples were also of highly 

uniform thickness.
6

List of Symbols, Abbreviations, and Acronyms

Ar argon

BST barium strontium titanate

CF4 carbon tetrafluoride

DC direct current

Di deionized

LOR list-off-resist

MIM metal-insulator-metal

mW/cm2 milliWatt per square centimeter

N2 nitrogen

Pt platinum

RF radio frequency

RIE reactive ion etching

rpm revolutions per minute

sccm standard cubic centimeter per minute

SEM scanning electron microscopy

SF6 sulfur hexafluoride

TE top electrode

UV ultraviolet
7

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