Measurement of mechanical and thermal

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Abstract In this study, mechanical and thermal proper- ties of the co-sputtered tungsten silicide (WSi) thin film were evaluated to consider the possibility of its use ...
Microsyst Technol DOI 10.1007/s00542-010-1109-6

TECHNICAL PAPER

Measurement of mechanical and thermal properties of co-sputtered WSi thin film for MEMS applications Bui Thanh Tung • Dzung Viet Dao • Toshiyuki Toriyama • Susumu Sugiyama

Received: 4 March 2010 / Accepted: 1 June 2010 Ó Springer-Verlag 2010

Abstract In this study, mechanical and thermal properties of the co-sputtered tungsten silicide (WSi) thin film were evaluated to consider the possibility of its use in MEMS applications. WSi film was prepared by cosputtering and basic micromachining processes, and its mechanical and thermal properties such as Young’s modulus, temperature coefficient of resistance (TCR), strain gauge factor, coefficient of thermal expansion (CTE) and thermal stress were studied. The measurement method was simple and efficient since only one test pattern was used for all measurements. At room temperature, the TCR, gauge factor, CTE and thermal stress were measured to be -670 ppm/°C, 2.8, 32 ppm/°C and 1.76 GPa, respectively. The dependence of these coefficients on temperature was also evaluated experimentally.

1 Introduction Owing to its low resistivity and high thermal and chemical stability, tungsten silicide (WSi) has been widely used in microelectronics technology as interconnection material, gate electrodes, and contact material. Recently, it has also been used as structural material for MEMS devices such as pressure sensors (Fujimori et al. 2007). WSi films have been prepared by many methods, for example, the reaction of a W thin film with Si at elevated temperatures (Siegal et al. 1989), chemical

B. T. Tung (&)  D. V. Dao  T. Toriyama  S. Sugiyama Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan e-mail: [email protected]

vapor deposition (Pauleau et al. 1989), deposition of W and Si by co-sputtering (Molarius et al. 1991), or sputtering of WSix (Washidzu et al. 1991). The properties of WSi thin films have been characterized and published. The properties of low-pressure chemical-vapor-deposited (PVCVD) WSi thin film were reported by Santucci et al. (1998) and the physical characteristics such as residual stress, morphology, and processing parameters of co-sputtered WSi were reported by (Ger and Brown 1995). Chemical-vapor-deposited metal layers generally show very high internal stress, so thin films fabricated by this method are not appropriate for use in sensing applications. Besides, WSi films prepared by chemical vapor deposition have very high levels of fluorine impurities (Ger and Brown 1995). In this study, sputtered WSi thin films were adopted as the structure layer. The physical properties of WSi films, including the electrical, thermal, and mechanical properties, were measured. The hardness and Young’s modulus of these samples were measured by using nanoindentation method. In order to evaluate the temperature coefficient of resistance (TCR) and strain gauge factor, a WSi test structure, i.e. a four-terminal WSi resistor in this study, was prepared by co-sputtering, electron beam (EB) lithography and reactive ion etching (RIE). In order to eliminate the influence of thermal stress on the TCR measurement, the free-standing WSi resistors were fabricated by releasing process or etching of buried SiO2 process. Both the TCR and strain gauge factor were measured on the same WSi resistor. The thermal stress in the fabricated WSi thin film was calculated from the resistance of the before and after released WSi resistors. Finally, the coefficient of thermal expansion (CTE) of the WSi thin film was extracted from the TCR of these resistors before and after the test structure is released.

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2 Test structure preparation The test structure is prepared by using the fabrication process as shown in Fig. 1. First, WSi thin film with thickness of 500 nm is deposited on top of 500 nm-thick SiO2 on a 750 lm-thick Si wafer by co-sputtering to obtain a Si/W atomic ratio of 2 (Fig. 1a, b). The mass density of WSi was measured to be 7.7 g cm-3, i.e. about 3.3 times higher than that of Si. The wafer is then annealed at 350°C to transform the sputtered layer to a more conductive stoichiometric form. Subsequently, four-terminal WSi resistors are patterned by EB lithography and RIE etching with 100 nm SiO2 hard mask (Fig. 1d, e). Next, the mask layer is removed (Fig. 1f). Finally, Al wire bonding is performed to make connection from electrodes to outer circuit for measurement. Figure 2b shows the microphotograph of the fabricated WSi resistor. The design of the four-terminal resistor with dimensions of 50 lm 9 2 lm 9 0.5 lm (l 9 w 9 tf) is shown schematically in Fig. 2a.

3 Measurements and results In this section, electrical conductivity, strain gauge factor and thermal properties of WSi will be measured on the same WSi four-terminal resistor as described in detail in the followings.

Fig. 2 Schematic view showing the dimensions of the resistor (a) and the top view of the fabricated WSi resistor (b)

3.1 Electrical properties The electrical conductivity of the fabricated WSi resistors was measured by using semiconductor analyzer (HP4155A). Figure 3 shows the I–V characteristics of 50 lm length WSi resistor. The linear I–V curve obtained demonstrates the Ohmic contact between WSi and Al wires. The electrical resistivity of the material is calculated based on resistance and dimensions of WSi resistor. The actual dimensions of WSi resistor were measured by using SEM (scanning electron microscopy) pictures. Finally, the resistivity of the WSi thin films was calculated to be about 0.8–1.0 mX cm. The resistance of WSi resistor was measured by four-point resistance measurement technique to eliminate the contact resistance of Al wires and contact pads. 3.2 Strain gauge factor measurement Fig. 1 Sample fabrication process: a deposition of SiO2, b cosputtering deposition of WSi and annealing, c deposition of SiO2 hard-mask, d SiO2 mask patterning by EB lithography and DHF etching, e WSi patterning, f SiO2 mask layer removing. g Side view of sputtered WSi and SiO2 thin films: WSi structural layer (500 nm); SiO2 sacrificial layer (500 nm); and Si substrate (750 lm)

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The strain gauge factor is evaluated by applying stress/ strain to the WSi resistor by using a cantilever bending method as shown schematically in Fig. 4a. The SiO2 layer in this case is utilized to transmit the stress/strain from the substrate to the WSi pattern layer.

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Fig. 3 I–V characteristics of WSi resistor

Fig. 4 Schematic of the experimental measurement of gauge factor. a Bending cantilever, b side view shows WSi resistor lies on top of the stress transmission layer SiO2

rF and eF are respectively the stress and strain in WSi resistor induced by the applied force F; E is Young’s modulus of WSi. RT and RrTF were measured directly. The Young’s modulus E of WSi was measured by using nanoindentation method to be 202 GPa. The stress rF is determined based on finite element method (FEM) using ANSYS software. An FEM cantilever model with a 500-nm-thick WSi layer on a 500-nm-thick SiO2 insulation layer on a Si substrate had been created. The Young’s modulus and Poisson’s ratio for Si and SiO2 were selected to be 168 GPa and 0.25, and 75 GPa and 0.17, respectively. The Poisson’s ratio of WSi was 0.15. Figure 5 shows the relative change of resistance induced by different strains at room temperature. The gauge factor is calculated from Eq. 1 to be 2.8, i.e. almost the same value as published by Schultes et al. (2005). However in that study, temperature dependence of gauge factor was not measured. The gauge factor is constant with respect to the strain in the measurement range. To investigate the dependence of gauge factor on temperature, measurement is performed in the temperaturecontrolled oven at different temperatures. At certain temperature, the resistances of WSi resistor were measured before and after loading. Changes of resistance upon temperature and strain gauge factor of WSi measured at different temperatures are shown in Figs. 6 and 7, respectively. As can be seen in Fig. 6, the resistance difference DRrTF (green triangulars) keeps constant in interested temperature range, while the resistance RT of the WSi resistor decreases with temperature, so the strain gauge factor (defined in Eq. 1) increases with temperature. The decrease of resistance RT of WSi resistor with temperature is due to the negative TCR (temperature coefficient of resistance), which will be presented in the next section.

WSi resistor is placed near the fixed end of the cantilever, which is fixed to the base using strain gauge epoxy. The cantilever has dimensions of 20 mm 9 3 mm 9 0.75 mm (length 9 width 9 thickness). By applying mechanical force to the free end of the cantilever, we can generate a stress or strain in WSi resistor. This measurement system is placed in temperature-controlled oven to evaluate the dependence of gauge factor on temperature. The strain gauge factor is defined as the change in resistance per unit strain and is calculated by the expression: r

GT ¼

DRTF RT

eF

¼

RrTF  RT E rF RT

ð1Þ

where RT and RrTF are respectively the resistances of WSi before and after external force is applied at temperature T;

Fig. 5 Resistance change upon application of strain

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Fig. 6 Resistance changes versus temperature

Fig. 8 WSi resistor after being released from Si substrate. a Schematic side view of released resistor. b SEM micrographs of released resistor

Fig. 7 Dependence of gauge factor on temperature

3.3 Measurement of temperature coefficient of resistance (TCR) The TCR represents for the change of resistance due to temperature change, and is expressed by: b¼

DRT R0

DT

¼

RT R0 R0

T  T0

ð2Þ

where R0 and RT are resistances of WSi resistor at temperature T0 and T, respectively. In this part, TCR and dependence of it on temperature will be measured. To avoid the affect of thermal stress caused by the thermal expansion mismatch between WSi film and substrate, the WSi resistor is released from substrate by vapor HF etching as shown schematically in Fig. 8a. When the temperature changes, the released WSi resistor can expand or contract freely, i.e. no thermal stress will be built in the resistor. Therefore, the measurement result of TCR will not be affected by thermal stress. Figure 8b shows the

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fabricated WSi resistor after being released from the Si substrate. The resistor had the length of 50 lm, width of 2 lm and thickness of 0.5 lm. The SiO2 only remains beneath the electrodes, where WSi area is large (more than 100 9 100 lm2). At position of WSi resistor, due to the small width (2 lm), SiO2 is removed completely during HF vapor process. Figure 9 shows the measurement results of resistance change versus temperature of the released and non-released WSi resistors. The difference between the resistances of released and non-released resistors is due to the thermal stress in the non-released resistor. The temperature range in this study is from 25 to 100°C. The dependence of TCR on temperature was calculated by Eq. 2 and plotted in Fig. 10 (the red lozenges). The average TCR was -670 ppm/°C, i.e. about four times smaller than those of W and Si, and nearly not depend on temperature in the interested range. In this figure, the TCR of the non-released resistor is also shown (blue squares) to compare with the actual TCR (the red lozenges). If the TCR was measured using non-released resistors, the result would be 15% higher than the actual. 3.4 Thermal expansion of WSi film Generally, temperature change leads to a change in the resistance of the non-released WSi resistor because of two factors: one is the change in resistivity due to temperature through TCR and the other is the resistance change resulting from a thermal expansion mismatch between the

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Fig. 9 Dependence of resistance on temperature

Fig. 11 Thermal expansion of sputtered WSi thin film

Fig. 10 Dependence of TCR on temperature

small compared with thickness of the Si substrate (ts = 750 lm), so they can be assumed to have the same in-plane displacement with the substrate (Kim et al. 1999; Klein 2000). Therefore, the CTE of the substrate is the CTE of Si, i.e. aS = 2.6 ppm/°C (Senturia 2001). The resistances Rr0 T and RrTT were measured directly as shown in Fig. 9 (blue squares). The TCR and strain gauge factor of WSi were measured in previous sections, therefore, from Eq. 4 the CTE of WSi thin film can be calculated as shown in Fig. 11. Thermal expansion coefficient decreases with temperature from 32 to 16 ppm/°C in the interested temperature range. At room temperature, it is a little bit higher than the result measured on co-sputtered WSi2 reported by Retajczyk and Sinha (1980) (i.e. a = 13.7 ppm/°C).

WSi film and the substrate. The difference in thermal expansion induces mechanical strain in the WSi layer. As a result, when the temperature changes, the total resistance change of the non-released WSi resistor can be expressed as the sum of thermal resistivity and thermal expansion effects as: DRrTT ¼ DRT þ DRr ¼ Rr0 T bDT þ Rr0 T eGT ¼ Rr0 T bDT þ Rr0 T ðaS  aT ÞDTGT r

aT  aS ¼

r

DRTT =R0 T DT

GT

r

¼

b

r

rT ¼ eT E ¼

r

ðRTT R0 T Þ=R0 T DT

GT

As shown in Fig. 9, at certain temperature T, the resistances of the non-released and released resistors are different. This difference courses by the thermal expansion mismatch between Si substrate and WSi film. This mismatch generates a stress in material called thermal stress. We can calculate this stress by following equation:

ð3Þ

or, b

3.5 Thermal stress in the WSi film

ð4Þ

where b is the TCR, Rr0 T and RrTT are the resistances of nonreleased WSi resistor at temperature T0 and T, respectively; aT and aS are CTE of the WSi thin film and substrate, respectively. It should be noted that in this study, the WSi and SiO2 thin films have thickness of 500 nm (tf, tf’ in Fig. 1), i.e. too

RrTT  RT E RT G T

ð5Þ

where rT is the thermal stress at temperature T; RrTT and RT are the resistances of the non-released and released WSi resistors, respectively. By using Eq. 5 we can obtain the residual stress in the WSi layer as shown in Fig. 12. WSi layer is under tensile stress at room temperature and this stress decreases with temperature. From these results, we can predict the temperature at which thermal stress equal to zero is about 440°C.

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Microsyst Technol Acknowledgments The authors would like to thank Mr. Tsukasa Fujimori, Mr. Hideaki Takano, and Dr. Yasushi Goto of Central Research Laboratory, Hitachi, Ltd., Japan for invaluable discussions and help with the fabrication of WSi samples.

References

Fig. 12 Thermal stress in the WSi layer at different temperatures

4 Conclusions In this study, the electrical, thermal and mechanical properties of the co-sputtered WSi thin film have been characterized experimentally. The sample was prepared by co-sputtering and basic micromachining processes. The measurement method was simple and efficient since only one four-terminal resistor is used for all measurements. Because of its high electrical conductivity, high elasticity, high mass density, high fatigue strength, low TCR, and especially, the compatibility of batch micro/nanofabrication, WSi can be a good material for MEMS devices such as micromechanical sensors and actuators. For example, WSi can be applied for MEMS capacitive inertial sensor, i.e. accelerometer or gyroscope. With mass density is 3.3 times higher than that of Si, while the Young’ modulus is just 1.2 time higher, the sensitivity of WSi capacitive inertial sensor would be three times higher than that of Si counterpart.

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