Ohmic contacts to single-crystalline 3C-SiC films for extreme-environment MEMS applications

Ohmic contacts to single-crystalline 3C-SiC films for extreme-environment MEMS applications

ARTICLE IN PRESS Microelectronics Journal 39 (2008) 1408– 1412 Contents lists available at ScienceDirect Microelectronics Journal journal homepage: ...

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ARTICLE IN PRESS Microelectronics Journal 39 (2008) 1408– 1412

Contents lists available at ScienceDirect

Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo

Ohmic contacts to single-crystalline 3C-SiC films for extreme-environment MEMS applications Gwiy-Sang Chung , Kyu-Hyung Yoon School of Electrical Engineering, University of Ulsan, San 29, Mugerdong, Namgu, Ulsan 680-749, Republic of Korea

a r t i c l e in f o

a b s t r a c t

Article history: Received 20 March 2008 Accepted 14 June 2008 Available online 8 August 2008

This paper describes the ohmic contacts to single-crystalline 3C-SiC thin films heteroepitaxially grown on Si (0 0 1) wafers. In this work, a TiW (titanium–tungsten) film was deposited as a contact material by RF magnetron sputter and annealed through the vacuum and rapid thermal anneal (RTA) process. Contact resistivity between the TiW film and the n-type 3C-SiC substrate was measured by the circular transmission line model (C-TLM) method. The contact phases and interface of the TiW/3C-SiC were evaluated with X-ray diffraction (XRD), scanning electron microscope (SEM) and Auger electron spectroscopy (AES) depth-profiles. The TiW film annealed at 1000 1C for 45 s with the RTA plays an important role in the formation of ohmic contact with the 3C-SiC film and the contact resistance is less than 4.62  104 O cm2. Moreover, the inter-diffusion at the TiW/3C-SiC interface was not generated during, before and after annealing, and was kept in a stable state. Therefore, the ohmic contact formation technology of single-crystalline 3C-SiC films by using the TiW film is very suitable for hightemperature micro-electro-mechanical system (MEMS) applications. & 2008 Elsevier Ltd. All rights reserved.

Keywords: Ohmic contact Single-crystalline 3C-SiC TiW Contact resistivity

1. Introduction Si-micro-electro-mechanical system (MEMS) technology has rapidly been developing as a high-value technology since the last decade. The integrated MEMS from Si micromachining to a compensating circuit can endure up to 120 1C due to the physical properties of Si and can also endure up to 300 1C by Si-on-insulator (SOI) [1]. Recently, the development of MEMS for the expected operational temperature over 500 1C is required at various industrial fields such as transportation machines, engine, space technology (ST), environment technology (ET) and power plants [2]. Among many wide-bandgap semiconductors, the study of silicon carbide (SiC) has been mainly focused on the nanoelectro-mechanical system (NENS) for information technology (IT) and bio technology (BT) industries as well as on MEMS for harshenvironment applications because of its high power, high frequency, high temperature, radiation-hard, corrosion-hard and superior electro-mechanical properties [3,4]. The hexagonal 4H- and 6H wafers are easily fabricated in 2 in. diameter, but are very expensive. Nevertheless, cubic b- or 3C-SiC still remains the only choice for low-cost and large-area applications in spite of a higher defect density. Cubic b- or 3C-SiC

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E-mail address: [email protected] (G.-S. Chung). 0026-2692/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.06.052

heteroepitaxially grown on Si wafers is very suitable for the M/NEMS technology using high temperature, high power, high frequency and bio-electrical devices because the electron mobility (1000 cm2/V s) of 3C-SiC is more excellent than its hexagonal SiC (4H-SiC: 950 cm2/V S, 6H-SiC: 450 cm2/V s) [5,6]. Therefore, we should precede metallization studies with regard to a thermally stable electrode formation to develop M/NEMS applications for IT, BT, ST and ET by using 3C-SiC with these excellent properties. Regarding the property of SiC, Schottky barrier height (SBH), given as a value of 0.1 eV, can be easily formed into good Schottky contacts, but it is very difficult to obtain a lower barrier height for good ohmic contact behavior. Therefore, it is necessary to develop a metal contact formation technique with lower contact resistivity [7]. In order to achieve better adhesion, chemical reactions occurred between the metal and SiC, which can lead to the diffusion of atoms through annealing. Moreover, this diffusion may lower the contact resistance as a result of additional doping of the SiC region close to the metal–SiC interface [8]. To date, a Ni line for n-type SiC and an Al line for p-type SiC have been used as ohmic contact applications of SiC, but aluminum, given its low melting point (660 1C), is not appropriate for high-temperature applications. Nickel has been reported to be a good ohmic contact because it forms silicide with SiC at elevated temperatures. However, the Ni-SiC reaction is also difficult to control to continuously yield lower contact resistance [9,10]. Recently, ohmic contact for high temperature has been studied

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using thermally stable metals such as W, Ti, and Ta. Of them, TiW is a suitable candidate for ohmic contact because it is thermally stable up to 2700 1C and no phase changes occur below this temperature [11]. Therefore, in this work, we evaluated electrical measurements as well as physical properties such as specific contact resistance, current–voltage (I–V) characteristic and interfacial reaction Titanium-based ohmic contacts to single-crystalline 3C-SiC according to annealing as a preceding study of development of the M/NENS device applications for high temperatures.

2. Experiments In this paper, single-crystalline 3C-SiC thin films are heteroepitaxially grown on Si (0 0 1) substrates by APCVD at a high temperature of 1350 1C using a single precursor such as hexamethyl disilane: {CH.36}Si2 (HMDS), which is easily decomposed at low temperature and has no risk of explosions [12]. Usually the linear transmission line model (L-TLM) method, which can easily calculate, is mainly used, but its process of manufacture is very complicated [13]. Therefore, these samples are patterned by the circular transmission line model (C-TLM) method, which results in a simple technology using only one mask level. Firstly, the free oxides formed on 3C-SiC thin films were removed in (1:3) HF:H2O solution. Later, to clean the surface acetone, methanol, and DI water were for 3 min, respectively. TiW thin films, of the Ti/W ratio of 10/90 vol/%, are deposited at about 2000 A˚ on single-crystalline 3C-SiC their films for ohmic contacts by an RF magnetron sputter. Table 1 summarizes the deposition conditions of TiW thin films. Fig. 1 shows a surface photography of the sample made in this study and the geometry for the contact resistivity measurement of TiW/3C-SiC. Firstly, the TiW/3C-SiC thin films were characterized by using X-ray diffraction (XRD), scanning electron microscope (SEM) and Auger electron spectroscope (AES) depth-

Tab1e 1 Deposition conditions of TiW thin films Parameter

Deposition conditions

Target RF power Substrate Target-substrate distance Working gas Substrate temp. Working pressure

TiW 200 diameter 200 W Single crystalline 3C-SiC film 8 cm Ar: 20 sccm Room temp. 5.0  102 Torr

1409

profile, respectively, to analyze interfacial mutuality diffusion and some cracks according to annealing. I–V characteristics of the circular TLM patterned TiW/3C-SiC thin films were also measured by an HP4155B semiconductor parameter analyzer to evaluate the specific contact resistance.

3. Results and discussion Fig. 2 shows that variation of the crystallization for TiW thin films is analyzed by XRD, which is set from 21 to 41 for an angle of incidence, and is injected into a value of 2y for a route of search to observe diffraction peaks. These results of XRD analysis for TiW thin films are shown in Fig. 2(a–c) using these conditions such as (a) before annealing and (b) after annealing at 1000 1C for 30 min with a vacuum furnace, and (c) at 1000 1C for 45 s with RTA, respectively. A Ti peak is much stronger than a TiW peak before annealing. By the annealing process, the Ti peak decreased, but a recombined TiW peak increased. Especially, in an RTA process, most Ti are recombined as TiW. The crack of TiW thin films deposited on 3C-SiC substrates by the annealing process was analyzed with SEM. The surface SEM images of TiW thin films were shown in Fig. 3(a) before annealing, (b) using the vacuum at 1000 1C, and (c) the RTA at 1000 1C. As SEM images, before and after annealing, there were no cracks on TiW deposited on SiC. Fig. 4 shows the AES depth-profile to analyze the interfacial mutuality diffusion and stability of TiW/3C-SiC before and after annealing. After annealing, it was found to be a stable state, which hardly changed in the interfacial TiW/3C-SiC. Fig. 4(a) shows little O2 on the surface layer, but Fig. 4(b and c) show that much O2 on TiW is diffused outside by annealing. Increasing Ti and O2 showed that the Ti and O2 of TiW react with each other at high temperatures. In case of exposure of TiW as the ohmic contact at high temperature, it is considered that the contact characteristic is deteriorated owing to the oxidation of Ti. Therefore, the formation of the oxidation film should be avoided for ohmic contact. We consider that TiW/3C-SiC can be used as ohmic contact applications if Au or Pt is used as anti-oxidation films [14]. Fig. 5 shows the I–V characteristics of TiW deposited on 3C-SiC thin films, which have heteroepitaxially grown on Si (0 0 1) substrates according to the annealing conditions. Before and after annealing, the TiW/3C-SiC contacts were observed as the Schottky and ohmic characteristics, respectively. Moreover, the RTA system was relatively more excellent than the vacuum furnace, as shown by one of the samples obtaining the best ohmic behavior at 1000 1C for 45 s. The results of the above SEM and AES exhibited good ohmic contact behavior owing to the recombination of the TiW thin films

r0 = 100 m r1' = 150 m r1 = 250 m r2' = 350 m r2 = 450 m r0 r1'

r1 r2'

r2

Fig. 1. (a) Surface photography of TiW electrodes and (b) the geometrical pattern for contact resistivity measurement of TiW/3C-SiC.

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Ti T iW

30

40

50 °2Theta

T iW

Ti T iW Ti 30

40

50

W 60

°2Theta T iW

T iW

Ti

Ti

30

40

50 °2Theta

W

60

70

Fig. 2. XRD variations of TiW thin films; (a) before annealing, (b) vacuum (1000 1C, 30 min) and (c) RTA (1000 1C, 45 s).

and improving an interfacial adhesion between TiW/3C-SiC according to the annealing. In this work, we observed a specific contact resistance of TiW/3C-SiC with the C-TLM method according to the annealing. The resistance (R1) between the first inside circle and the second circle,and the resistance (R2) between the second circle and the third circle were measured in Fig. 1(b), respectively. Then the specific contact resistance of the ohmic contact was calculated using the C-TLM method as given by Eq. (1) [15]   0   0   r2 r1 pc ¼ ln (1) R1  ln R2 ðr 0 Þ2 D r1 r0



lnðr 0

ar 0 Þ þ ð1=ar1 ÞðAðr 1

2 =r 1 Þ½ðEðr 0 Þ=

; r0

1 Þ=Cðr 1

; r0

Fig. 3. Surface SEM images of TiW thin films; (a) before annealing, (b) vacuum (1000 1C, 30 min) and (c) RTA (1000 1C, 45 s).

In this table, r0, r1, r0 1 and r0 2 are the radii of the various circles like in Fig. 1(b) and the value of D can be obtained by Eq. (2) as a function of the geometrical structure of these samples.

½2p=ðar 0 Þ2  0 0 0 0 0 1 ÞÞ  lnðr 1 =r 0 Þ½ð1=ar 1 ÞðAðr 1; r 1 Þ=Cðr 1; r 1 ÞÞ þ ð1=ar 2 ÞðAðr 2; r 2 Þ=Cðr 2; r 2 ÞÞ

(2)

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0.01 Before annealing Vaccum 800°C

W NO2 C KL 1 Ti LM3

Vaccum 1000°C RTA 1000°C (15sec.)

O KL1

RTA 1000°C (45sec.)

Currents [A]

Peak intensity (kcps)

SiLM1

0.00

Etch time (min.) -0.01 -5

C KL 1 Ti LM3

5

O KL1

RTA (sec.) 10-2

15s

30s

45s

Vacuum annealing RTA 1000°C

Etch time (min.)

SiLM2 W NO2 C KL 1

Peak intensity (kcps)

0 Voltages [V]

Fig. 5. I–V Characteristics of TiW/3C-SiC contact depending on annealing conditions.

Contact Resistivity (Ωcm2)

Peak intensity (kcps)

SiLM2 W NO2

Ti LM3

10-3

O KL1

10-4 800

− Vacuum annealing (°C)

1000

Fig. 6. Contact resistance variations of TiW/3C-SiC depending on annealing conditions.

Etch time (min.) Fig. 4. AES depth profiles of TiW/3C-SiC; (a) before annealing, (b) vacuum (1000 1C, 30 min) and (c) RTA (1000 1C, 45 s).

The a parameter is then defined as sffiffiffiffiffiffiffi Rsk a¼ pc

(3)

where Rsk is sheet resistance under the TiW contacted on poly 3C-SiC. The specific contact resistance of the interfacial TiW/3C-SiC according to the annealing is shown in Fig. 6.

The value of the specific contact resistivity decreased as the annealing temperature increases with a vacuum furnace and it was obtained as 5.85  104 O cm2 at 1000 1C. Moreover, the specific contact resistivity using the RTA system decreased as the annealing temperature increases and the lowest specific contact resistivity was 4.62  104 O cm2 for 45 s [12]. Therefore, it was found that good ohmic contact characteristics are necessary to obtain lower specific contact resistivity for improving the interfacial adhesion between TiW/3C-SiC at high temperature.

4. Conclusion This work focused on the ohmic contact of single-crystalline 3C-SiC thin films heteroepitaxially grown on Si substrates with

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APCVD methods using TiW thin films for MEMS applications at high temperature. The result of the XRD analysis was improved in the crystallization of TiW thin films according to the annealing. It was found to exist in stable thin films, which the interfacial TiW/3C-SiC has no both mutuality diffusion and any cracks by AES and SEM after annealing. The I–V curve also showed the ohmic contact characteristic using the RTA system and the vacuum furnace, but the RTA system was more excellent than the vacuum furnace. Especially, the specific contact resistivity measured by C-TLM methods was the lowest (4.62  104 O cm2) by the RTA system at 1000 1C for 45 s. Therefore, we propose the use of the electrode material of TiW/3C-SiC ohmic contact for high-temperature MEMS applications due to no interfacial mutuality diffusion and the thermally stable materials deposited by the RF magnetron sputter. Acknowledgments This research was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD)’’(D00177) which was conducted by the Ministry of Education, Science and Technology, and the grant No. B0009720 from the Regional Technology Innovation Program of the Ministry of Knowledge Economy. References [1] Gwiy-Sang Chung, Roya Maboudian, Bonding characteristics for 3C-SiC wafers with hydrofluoric acid for high temperature MEMS applications, Sensors & Actuators A: Physical 119 (2005) 599–604.

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