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MEMS Hotplates with TiN as a Heater Material
J.F. Creemer 1, W. van der Vlist 2, C.R. de Boer 2, H.W.
Zandbergen 1, and P.M. Sarro 2
1: Kavli Institute of NanoScience-HREM, 2: DIMES-ECTM
Delft University of Technology,
Delft, The Netherlands ***********************.nl
D. Briand and N.F. de Rooij
Institute of Microtechnology, University of Neuchâtel, Neuchâtel, Switzerland.
Abstract — Titanium nitride has been investigated as a heater material for hotplates and microreactors. TiN is CMOS compatible, and has a higher melting point (2950 ºC) than conventional heaters of Pt and poly-Si. For the first time, TiN is tested inside a conventional membrane of LPCVD SiN x . Two types of TiN are considered: high stress and low stress. Their performance is compared with that of Pt. The maximum temperature of TiN coils is 11% higher than Pt coils with the same layout and over 700 °C. For high-stress TiN, the TCR is almost constant and close to that of Pt, making it very suitable for temperature sensing. In the case of low-stress TiN the TCR becomes nonlinear and changes sign. The large differences between the nitrides are explained by the grain structure. Low-stress TiN contains many voids. They relax stress but strongly scatter the conduction electrons. The different grain structures are related to the sputtering parameters according to the Thornton model.
I. I NTRODUCTION
MEMS hotplates are often used for gas sensors because
of their low power consumption and cost-effective manufacturing. They are also used in membrane-type microreactors, as in the case presented here. Usually, a hotplate consists of a membrane of low-stress silicon nitride (SiN x ) with a heater coil or strip in the middle [1],[2]. The temperature can be derived from the resistance change of the coil. In general, the heaters are made of platinum (Pt) or polysilicon (poly-Si). Platinum, however, is not CMOS compatible, whereas poly-Si has an unstable resistivity above 550 ºC. Heaters for higher temperatures have been made of tantalum silicide (Ta 5Si 3) [3]. However, this material is not so widely available.
It is therefore attractive to consider titanium nitride (TiN) as an alternative [4][5]. TiN has a very high melting point (2950ºC) and can have a low electrical resistivity (20 µΩ cm [6]). Thin polycrystalline layers of TiN are widely used in CMOS metallisation processes as a diffusion barrier. For these reasons it has been proposed for heaters in general [4]. For hotplate heaters, TiN has two additional advantages. Its residual stress can be tuned over a wide range, increasing the strength of the hotplate. In addition, it has a very moderate heat conductivity (15 Wm -1K for bulk material). This promises low conductive losses through the connecting wires.
We have investigated heaters of high stress TiN (our standard material), low-stress TiN, and Pt with respect to resistivity, temperature coefficient of resistance (TCR), and maximum heater temperature. The properties of TiN are related to the deposition conditions and the morphology of
This research is supported by the Technology Foundation STW, Applied Science Foundation of NWO and the technology programme of the Ministry of Economic Affairs.
TABLE I.
M AJOR PARAMETERS FOR SPUTTERING OF T I N WITH HIGH AND LOW RESIDUAL STRESS .
Parameter High-stress Low-stress
N 2 partial pressure (Pa) 0.41 1.8 Total pressure (Pa) 0.53 2.3 Power (kW)
12 0.5 Substrate temperature (°C)
350 350 Bias voltage (V)
Membrane
Heater coil, bond pad Figure 1. Schematic cross section of the hotplates and top view of the
heater coil.
the grains.
II.
D ESIGN , FABRICATION , AND TESTING
The hotplates are made on silicon wafers in which square membranes of 1 mm wide are etched using a KOH solution; see Fig. 1 and 2. In the middle a heater coil is located of 0.33 mm wide. The coil has four contacts to enable accurate measurement of the resistance of the hot zone, as well as its power dissipation. The membrane consists of two layers of low-stress LPCVD SiN x , both 0.5 µm thick. The heater is sandwiched between them to be electrically and chemically isolated from the environment. Hotplates of such a design loose heat mainly by natural convection; radiation and conduction are much smaller [7].
The heaters are made by reactive sputtering of TiN in a Trikon Sigma d.c. magnetron reactor. The TiN has a thickness of 200 nm and is deposited on 10 nm Ti for a better adhesion to the SiN x . Two different types of TiN are sputtered: the standard one with high residual stress and one with low stress. The major sputtering parameters are given in
Table I. The stress in the TiN is determined from the wafer curvature with a Tencor FLX2908. The TiN layers are patterned by plasma etching with a chlorine-based chemistry similar to that used for etching aluminium. The contacts are opened by a plasma etch which is fluorine-based. In both etches, the selectivity is low and an end point detection mechanism is essential. The Pt heaters consist of 185 nm Pt sputtered on top of a 15 nm Ta adhesion layer. They are patterned using lift-off.
Examination of the grain structure of the TiN is done with an FEI CM30T transmission electron microscope (TEM). The electrical characterisation is done with an Agilent 4156C parameter analyser . The sheet resistance is measured on Van der Pauw structures. The spirals are heated up to failure by increasing the voltage, in 100 steps of 1 s The temperature is deduced from the resistance of Pt and from the power dissipation. It is assumed that the resistance of Pt increases linearly with the temperature, with a TCR of 2.08x10-3 °C -1. This value is based on previously calibrated hotplates with the same Pt layer [8]. It also corresponds with
-0.5
00.511.50
25
50
75
100
125
Power (mW)
∆R /R 0
Figure 4. Resistance change versus dissipated power, for heaters of high-stress TiN (TiN hs), platinum (Pt) and low-stress TiN (TiN ls).
Figure 2. Pt hotplate during operation under an optical microscope. The shadow is due to thermal buckling. The coil is 0.33 mm wide.
Figure 3. TiN hotplate during operation at its highest temperature,
with the illumination of the microscope turned off.
-0.5
00.511.50
200400
600800
Temperature (0
C)
∆R /R 0
Figure 5. Resistance change as a function of temperature, for heaters of high-stress TiN (TiN hs), platinum (Pt) and low-stress TiN (TiN ls).
the experience that thin film heaters start to glow visibly at 600 °C [9].
III.R ESULTS
After deposition the low-stress TiN has a reddish colour, and the high-stress TiN has a colour between gold and copper. The residual stress in the standard TiN is very high: -16.4 GPa, whereas it is only +0.1 GPa in the low-stress layers. The high stress is too much for many heaters. They suffer from delaminating during the deposition of the second LPCVD layer at 850 °C. The surviving structures show marked differences in the sheet resistances: 2.7 Ω/sq for the high-stress TiN and 0.17 kΩ/sq for the low-stress TiN. The sheet resistance of Pt is 1.4 Ω/sq.
During processing, TiN should be protected against oxidation. In particular, it should be avoided to heat TiN above 200°C in an oxygen-containing atmosphere. Also cleaning in an RCA SC-1 solution is not recommended. Finally, the TiN peels of in a KOH solution. The TiN is porous to this solution, which dissolves the underlying Ti layer.
By applying electrical power, TiN heaters can be brought to emit an intense yellow light; see Fig. 3. The Pt heaters can just be brought to a red glow in the dark. The related resistance changes, power dissipation, and temperatures are shown in Fig. 4 and 5. The TiN heaters can emit 40% more power than the Pt heater. The corresponding temperatures are approximately 720 °C and 650°C. Based on these values, the high-stress TiN has a TCR of 1.37x10-3 °C-1. The increase in resistance with temperature is fairly linear. The low-stress TiN, on the contrary, has a resistance which changes nonlinearly with the temperature. The change even becomes negative at high temperatures. After being heated to failure, the membranes with the TiN heaters are ruptured. In addition, there are blisters (delaminations) all over the heater coil. The Pt heaters fail with only a small blister in the centre.
The examination with the TEM reveals different grain structures for low- and high-stress TiN. As shown in Fig. 6, high-stress material consists of densely packed fibrous grains with a typical width of 10 nm. The structure corresponds to Zone T of the Thornton classification [10][11]. Low-stress material, on the other hand, has a porous structure of fibrous grains and contains many voids; see Fig. 7. Dark field images indicate that the grain width is equally around 10 nm. The grain structure corresponds to Zone 1 of the Thornton classification.
IV.D ISCUSSION
The colour of the TiN indicates that it is slightly rich of nitrogen [12]. This is coherent with the relatively high nitrogen pressure during deposition [13]. This also agrees with the resistivity of the high-stress material (57 µΩ cm), which is three times above the minimum value in literature (20 µΩcm [6]). This minimum is found only for
Figure 6. High-stress TiN layer seen in cross section by a TEM. The layer consists of long fibrous grains of approximately 10 nm wide. Figure 7. Low-stress TiN layer layer seen in cross section by a TEM. The white stripes are voids between the grains, shown enlarged in the inset.
stoichiometric layers, which are not reddish but have a yellow golden colour. Minimum resistivities are also found in materials which are free of oxygen contamination [13][14]. This can be reached by applying a bias voltage below -75V to the substrate. In the sputtering of our TiN no bias voltage has been applied.
The differences in properties between low- and high-stress TiN can be explained to a large extent from the difference in morphology of the grain structures. The voids in Zone-1 material relax residual stress. They also scatter electrons, thereby increasing the resistance. In addition, they can act as energy barriers for electrons traveling between grains. Lower stress, however, is also caused by a lower sputter power, decreasing the ion bombardment of the growing layer [15].
In general, the difference between the morphologies is introduced during the sputtering by a combination of gas pressure and substrate temperature [10][11]. A high gas pressure impedes surface diffusion of adatoms. This can be attributed to an increasing oblique component of the incident flux of gas atoms, developing into hemispherical incidence [11]. The limited surface diffusion increases shadowing effects and promotes the creation of voids.
The fact that TiN heaters reach higher temperatures than those of Pt confirms our expectations. These were based on the rule of thumb that above one-third of the melting point the grain boundaries of a material start to diffuse, thereby affecting the mechanical strength [3]. Failure of the TiN is probably due to the degradation of the SiN x, followed by a rupture of the membrane. The Pt heaters most likely failed due to electro-stress migration of the Pt atoms [16]. High-stress TiN has a linear TCR, which is interesting for sensing purposes. However, the resistance change of low-stress TiN is fluctuating and not so useful.
V.C ONCLUSION
Hotplates of TiN have been fabricated which heat up over 700 °C. The temperature is 11% higher than equal hotplates made of Pt. TiN with high residual stress has a good TCR for temperature sensing, in contrast to low-stress TiN. However, the high stress levels cause yield problems in the fabrication at present. Differences between high- and low-stress TiN are related to the grain structure as well as to the parameters of the sputtering.
A CKNOWLEDGMENT
We would like to thank the technical staff of the DIMES Technology Centre, Delft University of Technology, and the technical staff of ComLab, the joint IMT-CSEM clean room facility for their support in the fabrication of the devices. We thank Peter Swart for the photographs and the assistance with electrical measurements, and Vassili Svetchnikov for the TEM pictures. We are also grateful to Emile van der Drift, Frans Tichelaar, Patricia Kooyman, and Jan-Dirk Kamminga for the helpful discussions.
R EFERENCES
[1]J.M. Gardner, S.M. Lee, P.N. Bartlett, S. Guerin, D. Briand, and N.F.
de Rooij, “Silicon planar microcalorimeter employing nanostructured films”, Dig. Transducers 2001, Munich.
[2] D. Briand, A. Krauss, B. van der Schoot, U. Weimar, N. Barsan, W.
Göpel, and N.F. de Rooij, “Desingn and fabrication of high-temperature micro-hotplates for drop-coated gas sensors”, Sens.
Actuat. B 68 (2000) 223-233.
[3] A. Pollien, J. Baborowski, N. Ledermann, and P. Muralt, “New
material for thin film filament of micromachined hot-plate”, Dig.
Transducers 2001, Munich.
[4]P. de Moor, A. Witvrouw, V. Simons, and I. de Wolf, “The
fabrication and reliability testing of Ti/TiN heaters”, Proc. SPIE, Santa Clara, California, Sept. 1999, Vol. 3874 (1999) 284-293.
[5]J.F. Creemer, P.M. Sarro, M. Laros, H. Schellevis, T. Nathoeni, L.
Steenweg, V. Svetchnikov, and H.W. Zandbergen, “TiN for MEMS hotplate heaters”, Techn. Dig. Eurosensors XVIII, Rome, Sept. 2004, 706-707.
[6]N. Kumar, P. Pourrezaei, M. Fissel, T. Begley, B. Lee, and E.C.
Douglas, “Growth and properties of radio frequency reactively sputtered titanium nitride thin films”, J. Vac. Sci. Technol. A 5 (1987) 1778-1782.
[7] A. Pike and J. W. Gardner, “Thermal modelling and characterisation
of micropower chemoresistive silicon sensors”, Sens. Actuat. B 45 (1997) 19-26.
[8]L. Thiery, D. Briand, A. Odaymat, N.F. de Rooij, “Contribution of
scanning probe temperature measurements to the thermal analysis of micro-hotplates”, International Workshops on Thermal Investigations of ICs and Systems, Therminic 2004, Sophia Antipolis, France, September 2004, 23-28.
[9]R.M. Tiggelaar, “Silicon-based microreactors for high-temperature
heterogeneous partial oxidation reactions”, Ph.D. thesis, University of Twente, Enschede, The Netherlands, 2004.
[10]M. Ohring, “Material science of thin films, deposition and structure”,
Academic Press, San Diego, 2002.
[11]S. Craig and G.L. Harding, “Effects of argon pressure and substrate
temperature on the structure and properties of sputtered copper films”, J. Vac. Sci. Technol. 19 (1981) 205-215.
[12]M. Wittmer, “Properties and microelectronic applications of thin
films of refractory metal nitrides”, J. Vac. Sci. Technol. A 3 (1985) 1797-1803.
[13]G. Lemperière and J.M. Poitevin, “Influence of the nitrogen partial
pressure on the properties of D.C.-sputtered titanium and titanium nitride”, Thin Solid Films 111 (1984) 339-349.
[14]J.-E. Sundgren, “Structure and properties of TiN coatings”, Thin
Solid Films 128 (1985) 21-44.
[15] F. Vaz, P. Machado, L. Rebouta, P. Cerqueira, Ph. Goudeau, J.P.
Rivière, E. Alves, K. Pischow, and J. de Rijk, “Mechanical characterization of reactively magnetron-sputtereds TiN films”, Surf.
Coat. Technol. 174-175 (2003) 375-382.
[16] D. Briand, F. Beaudoin, J. Courbat, N.F. de Rooij, R. Desplats, and P.
Perdu, “Failure analysis of micro-heating elements suspended on thin membranes”, Microelectronics Reliability 45 (2005) 1786-1789.。