Measurement of Young’s modulus of Nickel silicide film by a surface profiler

Ming Qin Microelectronics Center, Southeast University, Nanjing 2 10096, P. R. China

C. Y. Yuen, M C Poon, W.Y. Chan Dept. of Electrical and Electronic Engineering, Hong Kong University of Science

and Technology, Kowloon, Hong Kong

Suggested area: Sensors and Actuators

Abstract: Young’s modulus of Nisi film fornied on crystal silicon is determined by measuring the deflection of tlze f i l rn as a ji~nction of the loaded force on the center of it. I n order to reduce the error caused by unknown Poisson’s ratio and tlze boundary of the structiire. Si.jNj f i lm Fvith a Young’s niocliilits of 370Gpa is used as a reference. The results sliovt. that Nisi formed at 35OoC has a Yoitrig’s niodulrts of 132GPn, kvhich is lower than that of nominl grown pol~silicon( -1 60Gpa) and Si.lN+ I t tneans that the N i s i iJ a promising structure material for MEMS application.

Introduction

Thin film materials have played an important role in- Micro-Electro-Mechanical systems (MEMS). Despite polysilicon, germanium, silicon carbide, and porous silicon film, silicide film is another promising material for MEMS application. Silicides have been used extensively in VLSI for reducing the series and contact resistance of the gate, source and drain regions [ I ] in the past decades. Among the silicides, nickel silicide has been regarded as one of the most promising silicides.

The Young’s modulus of silicide material is a key parameter during it‘s -MEMS application. Till now, the measurement of Young’s modulus

of silicide has not been reported. There are some techniques for measuring the Young’s modulus of thin film materials such as measuring the wafer warp through varying the coating thickness of the film or temperature, measuring the deflection of thin film under different pressure, or measuring the resonant frequency of vibrating structures. All these methods are relatively complicated and hard to achieve. Tai et al. proposed a new method that uses a step profiler to measure the

Young’s modulus of the materials [2], but an accurate simulation considering the boundary of the structure is needed. In this paper, a relatively simple method for measuring the Young’s modulus of thin film is introduced. This method uses a standard material that its Young’s modulus is known as a reference. Since the measured structure is same as that of reference, the error caused by the boundary of the structure can be eliminated.

Experiment

P-type <loo> 20-30 Q-cm Silicon wafers with a layer of thermal oxide are used as starting substrates. For forming a micro bridge at silicon wafer, 2000A thick nickel was deposited using high vacuum evaporation system after patterned the oxide on silicon. The base pressure in chamber was lxlO-*Torr but rose to lxlO-’Torr during the evaporation. In

order to obtain a low stress film on the wafer, the stress of the silicide has been investigated and found the silicide film has lowest value at about 335 “C annealing. For this experiment, rapid thermal annealing (RTA) was achieved for 10 min at 350 “C to form a low stress layer of silicide on the wafers. It has found that the formed silicide has the Nisi phase. Argon was used as a protecting gas. Nickel reacts with silicon and becomes nickel silicide. Nickel on the oxide still remains. After formation of silicide, the unreacted metal was removed using the solution of H2SO4 and H202 at 70°C for 5 min. The silicide is self-aligned with the pattern. For comparison, a layer of about 2750P\ thick Si3N4 is also formed at silicon wafers.

In order to achieve bridges on the wafer, the etching performance of nickel silicide is also checked and found that the TMAH doesn’t attack the film during the etching of silicon. So the bridges made by Nisi and Si3N4 are formed after 4 hours etching of TMAH at 85°C.

Measurement Theory

According to elastic theory, the deflection of fixed-fixed beam is given by

where F is load force on the center of the beam, L is the beam length, E is the Young’s modulus of the material, w and h are the width and thickness of the beam, respectively. So for the same structure, if we know ER, the Young’s modulus of reference material, DR and D can be measured by adding a fixed load on the center of the beam. Then, we have

Since the structure and load force is same for both reference and measured beam, this method avoids the error induced by stylus force and the boundary structures. In this experiment,

low stress nitride film was used as a reference material, its Young’s modulus is about 370GPa, which has been measured by Tai etc. P I .

Results and Discussions

From equation (l), the force added on the film is proportional to the deflection of the beam when the film is under elastic deflection. Figure 1 shows that the measured deflections of the silicide beam versus the stylus force. It was found that the beam is under elastic characteristic when the force varying from 6 mg to 20 mg. So it’s suitable we use 8 mg of force to measure the film. The measured deflections of silicide’ and Si3N4 film are shown in Figure 2. The thickness of Si3N4 beam measured by Nan0 Spec is 2750& and the thickness of silicide film measured by step profiler is 4421A. So using equation (2), the Young’s modulus of the nickel silicide formed on single crystal silicon at 350°C is 132GPa. This value is slightly higher than that of unannealed heavily doped polysilicon with phosphorus (123GPa) [2] but lower than that of normal polysilicon (160-170GPa)[3] and nitride film.

Figure 3 shows the SEM micrograph of the micro bridges formed by nickel silicide film on Si substrate. The bridge is 200 pm in length. Since TMAH doesn’t attack the silicide, this means that the Nisi can be used as a good structure material in Micro-Electro-Mechanical systems ( E M S ) .

Conclusion

The Young’s modulus of Nisi film formed at 350°C on crystal silicon is investigated by measuring the deflections of Nisi and Si3N4 beam using a surface profiler. The measuring method is quite simple but the precision depends on the Young’s modulus value of reference. The measured Young’s modulus of Nisi at 350 “C is about 132Gpa, which is lower than that of normal grown polysilicon and

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Si3N4. It means that the Nisi is a good structure Sensors, Actuators, Machines and Robots, material for MEMS application. NY, USA, (1990) 147.

W. N. SHARPE, JR., S. BROWN, G. C. References JOHNSON, W. KNAUSS, MRS,

Symposium Proceedings, 518 (1998) 57. 1.

2.

3.

R. MUKAI, S. OZAWA, H. YAGI, Thin Solid Films, 270 (1995) 567. Y. C. TAI, R. S. MULLER, Proceedings. EEE Micro Electro Mechanical Systems. An Investigation of Micro Structures,

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Figure 2 Measured deflection of Nisi (a) and Si3N4 (b) film at the stylus force of 8 mg generated by a surface Step Profiler.

Figure 3 SEM micrographs of micro-bridges made by low stress silicide film formed on single crystal Si substrate

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