Wet etching of sputtered tantalum thin films in NaOH and KOHbased solutions

S. Sood Æ R. Peelamedu Æ K. B. Sundaram ÆE. Dein Æ R. M. Todi

Received: 25 May 2006 / Accepted: 2 October 2006 / Published online: 18 November 2006� Springer Science+Business Media, LLC 2006

Abstract In this paper, a wet chemical etching tech-

nique to selectively etch tantalum thin film in sodium

hydroxide and potassium hydroxide based solutions

was developed. Tantalum thin films were deposited by

a DC-magnetron sputtering technique on silica and

yttria-stabilized zirconia (YSZ) substrates. After depo-

sition, the films were etched in hot NaOH/ H2O2 and

KOH/H2O2 based solutions with Au/Cr film as a hard

mask. The etch rate was studied as a function of

temperature and concentration of the etchants.

1 Introduction

As a highly stable refractory metal, Ta is chosen as a

thin film coating material in a variety of applications. It

also has high melting point, low toxicity, and excellent

chemical resistance to most chemicals [1, 2]. Tantalum

is an excellent electrode material for thin film capac-

itors and is one of the most promising barriers to

prevent highly diffusing copper from reacting with

underlying silicon [3, 4]. Tantalum-based capacitors

provide higher volumetric capacitance with high-reli-

ability characteristics over a wide range of tempera-

tures from –55�C to 125�C.[1]

Recently there has been much interest in the

fabrication of MEMS devices where Ta films exhibit

superior properties over other metals. As such, selec-

tive etching of Ta films is a technological issue in many

applications. The most common technique used for

etching Ta films is dry etching in low pressure plasma

reactors using a CF4/O2 mixture [5]. Etch rates of up to

6.0 lm/min have been achieved using this technique.

However, the difficulties with dry etching techniques

are that specialized equipment is needed and some

gases used in the process are very toxic and corrosive.

In the past, HF and HNO3 based chemistries have been

used to wet-etch Ta films [6, 7]. However, a HNO3–HF

mixture cannot be used for silicon and ceramic

substrates because it can result in damage to the

unprotected surface of the substrate due to the

presence of a relatively high amount of concentrated

hydrofluoric acid. Here for the first time, we report on

the wet etching studies of Ta thin films using hot

NaOH and KOH based solutions. This technique

provides the desired etching selectivity between the

Ta film and the ceramic substrate to pattern Ta thin

films into useful electrode structures. The etching

process was studied with different temperatures and

etchant concentrations for both NaOH–H2O2 and

KOH–H2O2 based solutions.

2 Tantalum film deposition and etching studies

For the etching studies, silicon and yttrium-stabilized

zirconia (YSZ) substrates were used. Prior to

S. Sood � R. Peelamedu � K. B. Sundaram �R. M. Todi (&)School of Electrical Engineering and Computer Science,University of Centeral Florida, 4000 Centeral Florida,Orlando, FL 32828, USAe-mail: rtodi@mail.ucf.edu

S. Sood � R. Peelamedu � K. B. Sundaram �E. Dein � R. M. TodiAdvanced Materials Processing and Analysis Center,University of Central Florida, Orlando, FL 32816, USA

J Mater Sci: Mater Electron (2007) 18:535–539

DOI 10.1007/s10854-006-9053-z

123

deposition, the substrates were thoroughly cleaned to

remove organic contaminants using the standard SC1

cleaning technique. The SC1 solution was prepared by

heating the solution of NH4OH and H2O in 5:1 ratio to

80�C. One part of H2O2 was then added to the solution

just prior to immersing the substrates. The substrates

were immersed in the solution and cleaned for 15 min.

This was followed by a DI water rinse for 5 min.

Tantalum thin films were DC sputtered using a

magnetron sputtering system at a fixed power of

200 W, constant pressure of 5 mTorr at 20 sccm of

Ar flow. The substrate temperature was maintained at

300�C during deposition. The sputtering chamber was

evacuated to a base pressure of 5 · 10–8 torr and

backfilled with Argon. Before the actual deposition,

pre-sputtering was done for 10 min to equilibrate the

target surface and remove any oxide and other

potential contaminants present. Films having a thick-

ness ranging from 150 nm to 5000 nm were successfully

deposited. The deposited films were then annealed

in-situ using the substrate heater set at 500�C to

remove any stresses induced in the Ta film during the

deposition. It was found that unannealed Ta films

tended to peel off the ceramic substrates due to

residual stress in the films.

Before etching, the films were patterned using a

photolithographic procedure as shown in the process

flow diagram (Fig. 1). Since both NaOH and KOH are

highly reactive etchants; we used a Au layer (~50 nm)

with an underlying seed layer of Cr (~30 nm) as a hard

mask. The Cr layer was used to improve the adhesion

of Au to the Ta film. Both Cr and Au films were

evaporated on Ta using a conventional filament evap-

oration system. The films were then patterned using

Shipley 1813 positive photo resist to open up the

windows for Ta etching. After etching, the Au was

removed using potassium iodide and iodine solution

while chrome was removed using a commercial chrome

etchant (CR-7). No under-cutting of the Cr/Au mask

layer was observed during etching. This is evident from

the vertical profile of the etched pattern shown in

Fig. 2 using a Veeco optical profilometer.

The patterned Ta samples were etched using both

NaOH and KOH based solutions with H2O2. Different

Fig. 1 Photolithographyprocess flow for fabricating aTa electrode pattern usingKOH/NaOH based solutions

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536 J Mater Sci: Mater Electron (2007) 18:535–539

weight percent solutions of NaOH and KOH were

prepared by mixing NaOH and KOH pellets in H2O.

The weight percent of a solution is calculated using the

following formula:

w/v% =mass of solute (g)

volume of solution (ml)� 100% ð1Þ

For each run, the prepared solution was carefully

pipetted into a Pyrex beaker and heated to the desired

etching temperature. H2O2 was then added to the

solution just before immersing the sample. Etching

studies were performed by changing temperature,

percentage of H2O2 in solution, and the mass percent

of NaOH/KOH in water.

The solution temperature was varied from 65�C to

105�C for both NaOH and KOH etchants. In each

case, the temperature was kept constant within 2�C.

The etch rates were measured using stylus profilome-

ter (Alpha Step). The etched films were further

Fig. 3 Average surfaceroughness (Ra) of thedeposited Ta film was 0.76 nmand peak-to-peak roughness(Rt) was 9.28 nm

Fig. 2 Optical profilometermeasurements for a 200 lmdiameter Ta dot using Au/Crmask gave less than 1%overall undercut

123

J Mater Sci: Mater Electron (2007) 18:535–539 537

characterized using a Veeco Wyko NT3300 optical

profilometer.

3 Results and discussion

The average surface roughness (Ra) measurements of

the deposited Ta films were of the order of 0.76 nm,

while the peak-to-peak roughness (Rt) measurements

were less than 9.28 nm as shown in Fig. 3 using the

optical profilometer. Figure 4 shows the etch rate of

Ta as a function of temperature using a 30% NaOH

and KOH with 10% H2O2 solution. Tantalum is

etched by the peroxide to form its mineral acid, in

this case tantalic acid (H2Ta2O6). This process is

accelerated at higher pH and elevated temperatures.

A strong base like KOH or NaOH speeds up the

dissolution. The NaOH based solutions had a higher

etch rate than KOH based solutions for same

percentage of H2O2 and at same temperature. The

etch rate exhibits an exponential increase with

temperature for NaOH and KOH based solutions

as shown in Fig. 4. The Ta etch rate as a function of

volume percentage of H2O2 in 30% NaOH and KOH

solution at 80�C is shown in Fig. 5. The etch rate has

a linear relation with H2O2 concentration in both

NaOH and KOH based solutions, with NaOH based

solutions showing higher etching rates. Figure 6

shows the etch rate as a function of weight percent-

age of NaOH or KOH in water at 85�C with 10%

H2O2. Figure 7 is a 3-D image of 50 lm wide Ta

electrode structures etched using NaOH and H2O2

that confirms negligible undercutting using this tech-

nique. The vertical walls of the etched Ta film in this

example exhibit the desirable smooth and straight

etch characteristics.

0

20

40

60

80

100

120

60 65 70 75 80 85 90 95 100 105

Temperature (oC)

Etc

h R

ate

usin

g N

aOH

(A

/sec

)

0

20

40

60

80

100

120

Etc

h R

ate

usin

g K

OH

(A

/sec

)

NaOH+H2O2 Soln. KOH+H2O2 Soln.

Fig. 4 Ta etch rate as afunction of temperature(using 30% NaOH and KOHwith 10% H2O2 in solution)

0

10

20

30

40

50

60

70

80

4 6 8 10 12 14 16 18 20 22

Percentage by Volume of H2O2 in NaOH and KOH Solutions

Etc

h ra

te u

sing

NaO

H(A

/sec

)

0

5

10

15

20

Etc

h R

ate

usin

gK

OH

(A/s

ec)

NaOH+H2O2 Soln. KOH+H2O2 Soln.

Fig. 5 Ta etch rate as afunction of volumepercentage of H2O2 in 30%NaOH and KOH solutions at80�C

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538 J Mater Sci: Mater Electron (2007) 18:535–539

4 Conclusion

Wet etching studies of Ta films on ceramic and silica

substrates have been performed with less than 2%

overall undercut having been achieved. The NaOH-

based solutions showed higher etch rates than KOH

based solutions with H2O2 as the reactive reagent.

Deep Ta film trenches with widths ranging from 10 lm

to 200 lm for use in MEMS components were

successfully fabricated using this technique.

References

1. W.D. Westwood, et al., Tantalum Thin Films (AcademicPress, London, 1975).

2. P. N. Baker, Invited Review. Thin Solid Films 14, 3–25 (1972)3. D. Ernur, et al., Microelectron. Eng. 64, 117–124 (2002).4. S. Kondo, et al., Jpn. J. Appl. Phys. 39 (Pt 1, No.11), 6216

(2000).5. R. Hsiao, et al., Thin Solid Films 304(1–2), 381 (1997).6. B. Wu, et al., United States Patent 6,329,299 (2001).7. H. Endo, et al., United States Patent. 4,446,115 (1984).

0

20

40

60

80

100

120

140

5 10 15 20 25 30 35 40 45 50 55

Percentage by weight of NaOH / KOH in H2O

Etc

h ra

te u

sing

NaO

H (

A/s

ec)

0

5

10

15

20

25

30

35

Etc

h ra

te u

sing

KO

H (A

/sec

)

NaOH+H2O2 Soln. KOH+H2O2 Soln.

Fig. 6 Ta etch rate as afunction of change in weightpercentage of NaOH andKOH in water (at 85� C with10% H2O2)

Fig. 7 Optical profilometer3-D image of 50 lm wide Taelectrodes patterned usingNaOH and H2O2 solution

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J Mater Sci: Mater Electron (2007) 18:535–539 539