Surface Analysis of Reactive Ion Etched PZT Thin Films 

in SF6 Plasma 

by Eugene Zakar 

ARL-TR-4284 September 2007 

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Army Research Laboratory 

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ARL-TR-4284 September 2007 

Surface Analysis of Reactive Ion Etched PZT Thin Films 

In SF6 Plasma 

Eugene Zakar 

Sensors and Electron Devices Directorate, ARL 

Approved for public release; distribution unlimited. 
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Surface Analysis of Reactive Ion Etched PZT Thin Films in SF6 Plasma 

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14. ABSTRACT 

Reactive ion etching of sol-gel deposited Pb(ZrO.52 Ti0.48)03 thin films was performed in SF6 plasmas. Etch rate was 

determined as a function of cathode power and chamber pressure, attaining a value of 65 nm/min at 300 W. Auger electron 

spectroscopy measurements revealed an excess Pb 10 nm thin layer on as-deposited film surfaces. X -ray photoelectron 

spectroscopy measurements showed the existence of ZrF4 and PbS04 species on etched surfaces, in addition to traces of S and 

F. These measurements also indicated that Ti is relatively easy to remove while Pb removal is the rate limiting step in the etch 

process. 

15. SUBJECT TERMS

Reactive ion etching, PZT, SF6

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Eugene Zakar 

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301-394-1628 

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iii 

Contents 

List of Figures iv

List of Tables iv

1. Introduction 1

2. Experiment 1

3. Results 2

4. Conclusion 7

5. References 8

Distribution List 10
iv 

List of Figures 

Figure 1. PZT etch rate as a function of power for pressures of 30 mT (■) and 50 mT (●), with 

100 sccm SF6 flow rate and 10 min. etch duration. ................................................................ 3 

Figure 2. XPS scan showing appearance of ZrF4 on PZT sample etched in SF6 plasma. ............. 4 

Figure 3. XPS scans of O1S data performed on (a) PZT control, and (b) SF6 plasma etched 

samples.................................................................................................................................... 5 

Figure 4. Pb concentration of 0.25 μm thick PZT sample determined from AES measurements 

as film was sputtered through to the Pt layer with 8 kV, 12 mA, Ar ion beam over a 200 μm 

square. ..................................................................................................................................... 6 

List of Tables 

Table 1. XPS measurements of relative atomic percentages of Pb, Zr, and Ti for control and 

etched PZT samples. ............................................................................................................... 7 
1 

1. Introduction 

Lead zirconate titanate [Pb(ZrxTi1-x)03] (PZT) thin films usher in a new generation of sensor, 

actuator, memory, and microelectromechanical systems (MEMS) due to their superior 

piezoelectric properties. To incorporate/integrate these materials into silicon IC technology it is 

necessary to develop anisotropic dry etch processes to etch desired, precisely accurate and defect 

free PZT structures in the micrometer size range. One of the widely used dry etching processes 

is halogen plasmas - chlorine and fluorine based gases which have shown high potential to 

achieve the desired results (1-14, 20). However, selection of fluorine (1) or chlorine (13) 

chemistry requires extensive experimental analysis to understand the basic reaction chemistry. 

Most recently a mixture of fluorine and chlorine gases have been proposed for a clean etch 

profile of PZT films with a fast etch rate using high plasma density sources ECR (21), EDCR 

(22), and ICP (23, 24). 

One of the simplest ways to understand the reactions underlying the plasma etching mechanism 

is the study of etching by-products on the surface of PZT. Analysis of PZT surfaces etched in 

various single halogen (e.g: Cl and F) molecules has been performed in the past using techniques 

such as x-ray photoelectron spectroscopy (XPS) (3, 8-10, 13, 15) to determine the presence and 

composition of surface reactant species. Recently SF6 plasmas have been used successfully to 

etch PZT (5, 6, 25-30), since it is a non-toxic, stable gas and is widely used in IC manufacturing. 

To date, very little surface studies have been reported on PZT etched with SF6. We report here 

about XPS, Auger electron spectroscopy (AES), and etch rate measurements performed on solgel deposited PZT films reactive ion etched (RIE) in SF6 plasmas. 

2. Experiment 

Sol-gel Pb(Zr0.52Ti0.48)03 solutions were spin coated onto the platinized Pt(170 nm)/Ta(20 

nm)/Si02/Si wafers followed by crystallization using rapid thermal annealing at 700 °C to get the 

thin films 250-1000 nm. This is a very common structure used for PZT MEMS devices, where 

the Pt layer is the bottom electrode. Etching was performed in a PlasmaTherm 720 Shuttlelock 

RlE system with a standard parallel plate reactor design. Samples were patterned with AZ4330 

photoresist and mounted on a liquid cooled cathode surrounded by an Ardel annular ring 

electrode shield material to enhance etch uniformity. XPS measurements were performed using 

a Physical Electronics model PHI 5800 system. AES measurements were performed using a 

Physical Electronics model PHI 600 scanning Auger microprobe. 
2 

3. Results 

Constituents of PZT are comprised of heavy metals (Pb, Zr, Ti), and produce halide byproducts 

having low volatilities under normal SF6 RIE conditions which may redeposit attributing to 

physical sputtering rather than chemical etching (16). Since etching of PZT films to low energy 

(30-40 eV) was found to produce etch yields that were greater than the expected sputter yields 

under the same conditions (16), this indicates that some chemical reaction may also be occurring 

in the fluorine plasmas. Figure 1 shows PZT etch rate as a function of cathode power for 

30 mTorr and 50 mTorr chamber pressures. As power was increased from 100 W to 300 W, the 

etch rate increased from 15 nm to 65 nm/min, cathode bias increased from 25 V to 225 V. The 

rate increase with power is probably due to higher associated ion bombardment energies, which 

induce ion-assisted surface effects such as breaking of chemical bonds. This results in enhanced 

formation of reactant species, and the lowering of the desorption energy of these species. Etch 

rate increase could also be partly due to increased production of reactant fluorine species, 

although this is probably a secondary effect. Etch rate also increased as pressure was reduced. 

Since induced bias voltage is a measure of ion bombardment energy, the etch rate increase from 

the lower operating pressures further indicates that the etch process is dependent on ion 

bombardment effects. 
3 

Figure 1. PZT etch rate as a function of power for pressures of 30 mT (■) and 

50 mT (●), with 100 sccm SF6 flow rate and 10 min. etch duration. 

Figure 2 shows XPS data obtained on a PZT sample surface etched in SF6 at a cathode power of 

300W for 30 mT chamber pressure. The peak shown at 685.5 eV was not present prior to 

etching and corresponds to the formation of ZrF4. This is a surface reactant species that would 

be expected to appear when etching PZT in fluorine-based plasmas. However, no evidence was 

seen of Pb or Ti based reactant species such as PbF2 or TiF4. Figure 3(a) shows the oxygen 01S

peak at 531 eV from a PZT control sample, while figure 3(b) shows the same scan from a PZT 

sample etched in SF6. The figure 3(b) data can be resolved into the sum of two peaks centered at 

530.7 eV and 532.5 eV as shown. The peak at 530.7 eV corresponds to TiO2 and ZrO2 species, 

while the additional 532.5 eV peak indicates the presence of PbSO4, a nonfluorine based 

chemical compound. 
4 

Figure 2. XPS scan showing appearance of ZrF4 on PZT sample etched in SF6 plasma. 
5 

(a) 

(b) 

Figure 3. XPS scans of O1S data performed on (a) PZT control, and (b) SF6 plasma etched samples. 

AES measurements were performed on a control 0.25 μm PZT film sample sputtered with a 

8 kV, 12 mA, Ar ion beam over a 200 μm square. Figure 4 shows the Pb atomic percent 

concentration as the film was sputtered through to the Pt under layer after about a 400s time 

duration. The film exhibited a Pb-rich surface region having an estimated thickness of 10 nm 
6 

and a concentration more than twice that of the bulk region. The Pb-rich surface regions similar 

to that observed here have been reported by others on PZT layers (17-19), and are thought to be 

an inherent result of thermal cycling of the deposited material, independent of the specific film 

deposition process. This non-ferroelectric surface region artifact can result in a reduced 

dielectric constant for the sample and may be detrimental to the operation of devices fabricated 

from PZT films. 

Figure 4. Pb concentration of 0.25 μm thick PZT sample determined from AES 

measurements as film was sputtered through to the Pt layer with 8 kV, 12 mA, 

Ar ion beam over a 200 μm square. 

It is instructive to compare the Pb, Zr, and Ti components upon etching to provide an 

understanding of the etching mechanism. The relative atomic percentages of Pb, Zr, Ti, S, and F 

were estimated from XPS analysis for a control and an etched sample. The etched sample 

exhibited residual sulfur and fluorine compositions of 4.7% and 4.0%, respectively. Table 1 

shows the relative atomic percentages of Pb, Zr, and Ti for both control and etched samples. 

Prior to performing XPS measurements on the control sample, the Pb-rich surface region was 

first sputtered through using an Ar ion beam so that a more valid initial surface composition 

could be obtained for comparison purposes. Table 1 shows that while the relative atomic 

percentages of Zr and Ti both decreased upon etching, the percentage of Pb increased, suggesting 

that Pb is the most difficult PZT component to remove. Other workers (9,13,15,16) have also 

reported that the removal of Pb is the rate limiting step for PZT etching in fluorine based 

plasmas. The formation of PbSO4 during the etching process may add to the difficulty of Pb 

removal. In addition, table 1 shows that both the Ti/Pb and Ti/Zr ratios decreased upon etching, 

indicating that the Ti component of PZT is more easily removed than Pb or Zr in SF6 plasmas. 

Similarly, Lin, et al. (3) found that Ti by-products were completely volatilized for RIE of PZT in 
7 

CHF3 plasmas, while the fluorides of Pb and Zr were more difficult to remove. The above 

results are consistent with the fact that PbF2 has a melting point of 855 °C, while ZrF4 and TiF4

sublime at 600 °C and 284 °C, respectively, (16) leading to the expectation that for PZT the Pb 

species would be most difficult to etch with the Ti species being most readily etched in SF6

plasmas. In contrast, An, et al., (l0) found that the Pb component etched faster than Zr and Ti in 

chlorine based plasmas. 

Table 1. XPS measurements of relative atomic percentages of Pb, Zr, 

and Ti for control and etched PZT samples. 

Pb Zr Ti 

Control 10.1 10.9 8.5 

SF6 Etched 10.9 10.4 4.5 

4. Conclusion 

XPS and AES were used to analyze surface reaction and the by-products of the PZT RIE in SF6. 

XPS measurements revealed the presence of ZrF4 and PbSO4, along with residual amounts of S 

and F. XPS measurements also indicated that Ti is readily removed from the PZT surface, while 

Pb removal is the rate limiting step in this etching process. AES measurements showed the 

presence of a Pb-rich surface region approximately 10 nm thick on as-deposited PZT material. 

RIE rates of PZT in SF6 plasma measured up to 65 nm/min at 300 W power. Conventional 

photoresist masking materials were used during the patterning and etch process, but more 

durable materials such as Pt, Ni, or Si3N4 can be used as a mask to improve the selectivity, etch 

rate, and may favorably influence the sidewall angle profiles of the etched PZT. There is a need 

to carefully monitor and control the amount of residual Pb and S that can redeposit on etched 

sidewalls of fabricated PZT thin film capacitor micro cells for Ferroelectric Random Access 

Memory (FRAM) applications. 
8 

5. References 

1. Archard, H.; Mace, H.; Peccoud, L. Microelectron. Eng. 1995, 29, 19. 

2. Zeto, R.; Rod, B. J.; et al. Proc. 11th IEEE Int. Symp. on Applications of Ferroelectrics

1998, 8, 89. 

3. Lin, Y.; Liu, Q.; Tang, T.; Yao, X.; Huang, W. Jap. J. Appl. Phys. 2000, 39, 320. 

4. Van Glabbeek, J. J.; Spierings, G.A.C.M.; et al. Mat. Res. Soc. Symp. Proc. 1993, 310, 127. 

5. Wang, S.; Li, X.; Wakabayashi, K.; Esaski, M. J. Am. Ceram. Soc. 1999, 82, 1339. 

6. Bale, M.; Palmer, R. E. J. Vac. Sci. Technol. 1999, A17, 2467. 

7. Pan, W.; Thio, C. L.; Desu, S. B. J. Mater. Res. 1998, 13, 362. 

8. Chung, C. J. Vac. Sci. Technol. 1998, B16, 1894. 

9. Pan, W.; Desu, S. B.; Yoo, 1.K.; Vijay, D. J. Mater. Res. 1994, 9, 2976. 

10. An, T.; Park, J.; Yeom, G.; Chang, E.; Kim, C. Vac. Sci. Technol. 2000, A18, 1373. 

11. Chung, C.; Kim, C. Jap. J. Appl. Phys. 1997, 36, 2747. 

12. Yokoyama, S.; Ito, Y.; Ishihara, K.; et al. Jap. J. Appl. Phys. 1995, 34, 767. 

13. Vijay, D.; Desu, S. B.; Pan, W. J. Electrochem. Soc. 1993, 140, 2635. 

14. Saito, K.; Choi, J.; Fukuda, T.; Ohue, M. Jap. J. Appl. Phys. 1992, 31, L1260. 

15. Lin, Y.; Liu, Q.; Tang, T.; Yao, X. Appl. Surf. Sci. 2000, 165, 34. 

16. Lau, W. M.; Bello, 1.; Sayer, M.; Zou, L. Appl. Phys. Lett. 1994, 64, 300. 

17. Dana, S.S.; Etzold, K.F.; Clabes, J. J. Appl. Phys. 1991, 69, 4398. 

18. Vasco, E.; Bohme, O.; Roman, E.; Zaldo, C. Appl. Phys. Lett. 2001, 78, 2037. 

19. Qian, Z.; Xiao, D.; Zhu, J.; Li, Z.; Zuo, C. J. Appl. Phys. 1993, 74, 224. 

20. Efremov, A. M.; Kim, D. P.; et al. Plasma Chem. 2004 24 (1), 13-28. 

21. Baborowski, J.; Muralt, P.; et al. Integrated Ferroelectrics 2000 31 (1-4), 261-271. 

22. Mace, H.; Achard, H.; Peccoud, L. Microelectronic Eng. 1995 29 (1-4), 45-48. 

23. Chung, C. W.; Byun, Y. H.; Kim, H. I. Korean J. Chem. Eng. 2002 19 (3), 524-528. 

24. Efremov, A. M.; Kim, D. P.; Kim, C. I. Thin Solid Films 2005 474 (1-2), 267-274. 
9 

25. Tang, T.A.; Chen, Z.; Li, N.; et al. Ferroelectrics 1999, 232 (1-4), 927-932. 

26. McLane, G.; Polcawich, R.; et al. Integrated Ferroelectrics 2001, 37 (1-4), 397-404. 

27. Bale, M.; Palmer, R. E. J. Vac. Sci. Tech. (B) 2001, 19 (6), 2020-2025. 

28. Kim, S. J.; Hong, K. I.; Choi, D. K. Jap. J. Appl. Phys. 2003, 42 (3), 1475-1478. 

29. Shibata, T.; Unno, K.; et al. Sensors and Actuators (A) 2004, 114 (2-3), 398-405. 

30. Whatmore, R. W.; Landi, S.; Shaw, C. P.; et al. Ferroelectrics 2005, 318, 11-22. 
10 

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