Realization and characterization of free-standing surface-microstructures based on Ta-Si-N films are presented. Due to their significant physical and chemical properties, such ternary films are promising candidates for application in microelectromechanical devices

de Rooij, N. F.

Dommann, A.

Grétillat, M.-A.

Linder, C.

Nicolet, M. A.

Staufert, G.

English

Nicolet, M.-A.

Caltech Authors - Main

Surface-micromachined Ta-Si-N beams for use in micromechanics

J. Micromech. Microeng. 8 (1998) 88–90. Printed in the UK PII: S0960-1317(98)86816-2
Surface-micromachined Ta–Si–N
beams for use in micromechanics
M-A Gre´tillatyk, C Linderz, A Dommannz, G Staufertz,
N F de Rooijy and M-A Nicoletx
y Institute of Microtechnology, University of Neuchaˆtel, CH-2007 Neuchaˆtel,
Switzerland
z Institut fu¨r Mikrosystemtechnik, Neu-Technikum Buchs, CH-9471 Buchs,
Switzerland
x California Institute of Technology, Pasadena, CA 91125, USA
Received 12 August 1997, accepted for publication 2 December 1997
Abstract. Realization and characterization of free-standing surface-microstructures
based on Ta–Si–N films are presented. Due to their significant physical and
chemical properties, such ternary films are promising candidates for application in
microelectromechanical devices.
1. Introduction
Amorphous Ta–Si–N thin films have been proven to
successfully meet the requirements for use as diffusion
barriers for metal/silicon contacts in microelectronics [1].
In addition, the resistivity of such Ta–Si–N films is a
few hundred  cm for nitrogen concentrations less than
50% [2], thus filling the gap between metal conductors
and doped polysilicon. Furthermore, the amorphous
structure of this new material may hamper fatigue effects
as often observed in metals. The fabrication of x-
ray amorphous Ta–Si–N microbeams using a sacrificial
Al layer has been shown previously [3]. This paper
describes an alternative IC-compatible process employing
phosphorus-doped silicon dioxide (PSG) as a sacrificial
layer. Furthermore, etching characteristics of Ta–Si–
N are investigated, i.e., the resistance in wet etchants
as well as the patterning by dry etching. Finally, the
electromechanical behaviour of Ta–Si–N beams integrated
together with substrate diffusions is demonstrated.
2. Experimental details
The Ta–Si–N films have been deposited by reactive
sputtering in an Ar/N2 plasma (5% N2 partial gas pressure)
employing a Ta5Si3 target. In order to decrease the
compressive stress of the as-deposited films, annealing at
450 C is carried out [3]. An example of a fabricated
free-standing Ta–Si–N microstructure is shown in figure 1.
The PSG layers have been produced by chemical vapour
deposition (CVD) at 350 C and densified at 625 C. A
lateral etch rate of about 0.4 m min−1 is achieved for the
sacrificial PSG in buffered hydrofluoric acid (BHF 7:1).
k Phone: +41 32 7205 121; fax: +41 32 7205 711;
email: marc.gretillat@imt.unine.ch
Figure 1. A SEM of a Ta–Si–N microactuator beam
structure; the beam is 300 m long and 80 m wide. The
underetch holes are 10 m in diameter and the tethers are
20 m wide.
Table 1. The chemical resistance of Ta–Si–N.
Chemical Time
BHF [7:1] more than 60 min
HNO3 (100%) at least 10 min
H2SO4 + H2O2 at least 10 min
H3PO4 + HNO4
(aluminium etchant) more than 100 min
As indicated in table 1 etch tests revealed that Ta–Si–N
resists BHF (7:1) for more than one hour. This is essential
for the sacrificial layer technology where etching times of
this order are required. Furthermore, Ta–Si–N successfully
withstands wet etching in HNO3 (100%) and H2SO4=H2O2,
i.e., no thickness reduction or degradation such as strong
oxidation was observed. Thus, standard cleaning and resist-
stripping steps using the above acid mixtures do not attack
Ta–Si–N layers.
The patterning of Ta–Si–N films is usually performed
0960-1317/98/020088+03$19.50 c© 1998 IOP Publishing Ltd
Surface-micromachined Ta–Si–N beams
Table 2. Ta–Si–N dry etching: process parameters and results.
Gases C2ClF5=SF6 SF6=O2
Mask positive photoresist positive photoresist
Pressure (mbar) 0.1 0.2
Power (W) 100 125
Etch rate (nm min−1) 70 140
Figure 2. A SEM close-up of the anchor pad (right-hand
side) of a 300 m long and 20 m wide Ta–Si–N beam
actuator.
by dry etching [4]. As shown in table 2, reactive ion
etching (RIE) using SF6=O2 and C2ClF5=SF6 gases has
been applied. The chlorine/fluorine gas mixtures resulted
in better uniformity than the fluorine (oxygen) gases. The
sidewall profiles of a Ta–Si–N structure as etched in
a C2ClF5=SF6 plasma are illustrated in figure 2. The
remaining parts at the edges can be suppressed by an
appropriate overetching. Choosing appropriate process
parameters for power and pressure, etch rates of more than
100 nm min−1 are achieved.
3. Application and discussion
By adding substrate diffusions as counter-electrodes
under the Ta–Si–N microbeams as sketched in figure 3,
electrostatically driven prototype actuators have been built.
The transfer function of these devices under vacuum
has been measured with an optical interferometer. The
output signal of the latter has been fed into a gain-
phase analyser. Figure 4 shows the characteristics for
such a clamped–clamped beam. Resonance frequencies of
113 kHz and 140 kHz were measured for 350 m and
300 m long beams having a thickness of 2 m. As
can also be seen from figure 4, these devices exhibit a
relatively high quality factor of about 1000 in vacuum.
Thus, the resonance behaviour of the Ta–Si–N structures is
promising for application in micromechanical devices such
as relays [5]. Compared with polysilicon, the amorphous
Ta–Si–N has similar mechanical properties but a higher
electrical conductivity. In addition, a comparable lifetime
is expected.
4. Conclusion
Free-standing Ta–Si–N microbeams, employing CVD
silicon dioxide as sacrificial layer, have been demonstrated.
Figure 3. Schematic cross section of a surface
micromachined Ta–Si–N actuator with an electrostatic
drive. CVD silicon dioxide serves as the sacrificial layer.
Figure 4. Measured transfer characteristic of a 350 m
long beam actuator in vacuum; the resonance frequency
reveals a quality factor of about 1000.
The outstanding physical properties of the Ta–Si–N
structures point out the attractiveness of this material
for use in microelectromechanical components such as
resonators or switches. Moreover, the resistance of Ta–
Si–N to chemicals allows for application in an aggressive
environment. Future work will focus on the mechanical
property characterization.
References
[1] Nicolet M-A 1995 Ternary amorphous metallic thin films as
diffusion barriers for Cu metallization Appl. Surf. Sci. 91
269–76
89
M-A Gre´tillat et al
[2] Kolawa E, Molarius J M, Nieh C W and Nicolet M-A 1990
Amorphous Ta–Si–N thin-film alloys as diffusion barrier
in Al/Si metallizations J. Vac. Sci. Technol. A 8 3006–10
[3] Linder C, Dommann A, Staufert G and Nicolet M-A 1996
Amorphous metals as new materials for transducer
applications Proc. 10th Eur. Conf. Solid-State Transducers
(Eurosensors X) No C2-2
[4] McLane G F, Casas L, Reid J S, Kolawa E and Nicolet
M-A 1994 Reactive ion etching of TaSiN diffusion barrier
in CF4 C O2 J. Vac. Sci. Technol. B 12 2352–5
[5] Gre´tillat M-A, Thie´baud P, de Rooij N F and Linder C 1994
Electrostatic polysilicon microrelays integrated with
MOSFETs IEEE Micro Electro Mech. Syst. Workshop
pp 97–101.
90


Surface-micromachined Ta–Si–N beams for use in micromechanics

Amorphous metals as new materials for transducer applications

Amorphous Ta–Si–N thin-film alloys as diffusion barrier in Al/Si metallizations

Gre´tillat M-A, Thie´baud P, de Rooij N F and Linder C 1994 Electrostatic polysilicon microrelays integrated with MOSFETs IEEE Micro Electro Mech. Syst. Workshop pp 97–101.

Kolawa E, Molarius J M, Nieh C W and Nicolet M-A 1990 Amorphous Ta–Si–N thin-film alloys as diffusion barrier in Al/Si metallizations J. Vac. Sci. Technol. A 8 3006–10

Linder C, Dommann A, Staufert G and Nicolet M-A 1996 Amorphous metals as new materials for transducer applications Proc. 10th Eur. Conf. Solid-State Transducers (Eurosensors X) No C2-2

McLane G F, Casas L, Reid J S, Kolawa E and Nicolet M-A 1994 Reactive ion etching of TaSiN diffusion barrier in CF4 C O2 J. Vac. Sci. Technol. B 12 2352–5

Nicolet M-A 1995 Ternary amorphous metallic thin films as diffusion barriers for Cu metallization Appl. Surf. Sci. 91 269–76 M-A Gre´tillat et al

Reactive ion etching of TaSiN diffusion barrier in CF4

Ternary amorphous metallic thin films as diffusion barriers for Cu metallization

https://authors.library.caltech.edu/390/1/GREjmm98.pdf

Surface-micromachined Ta–Si–N beams for use in micromechanics

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