MEASUREMENTS OF MECHANICAL PROPERTIES OF MICROFABRICATED THIN FILMS

Transcription

1 MEASUREMENTS OF MECHANCAL PROPERTES OF MCROFABRCATED THN FLMS H. Ogawa, K. Suzuki**, S. Kaneko", Y. Nakano**, Y. shikawa and T. Kitahara Mechanical Engineering Laboratory, AST, MT, Department of Advanced Machinery Namiki 1-2, Tsukuba, baraki, 35 Japan *Olympus Optical Co., Ltd., Research Department Kuboyama-cho 2-3, Hachioji, Tokyo, 192 Japan **Shonan nstitute of Technology Nishikaigan , Tsujido, Fujisawa, Kanagawa, 251 Japan ABSTRACT Mechanical properties of titanium thin films of.5 fim thickness, microfabricated by magnetron sputtering, were measured by using a novel tensile machine. These thin films are difficult to handle because they are markedly fragile, so the thin film specimens were fabricated by using semiconductor manufacturing technology in silicon frames to protect them. The test section of these specimens was 3,um in width and 1.4 mm in length. By gripping the thin film specimen with a new device using a micrometer, it could be mounted on the tensile machine easily. The stress-strain diagrams of thin films were measured continuously in the atmosphere at room temperature. Tensile tests were conducted on specimens of 1. to 1.4 mm gauge length in order to examine the effect of gauge length on the measurements of mechanical properties. Measurements of Young's modulus, tensile strength and breaking elongation were independent of gauge length for our range of measurements. The experimental results indicated that the titanium thin films had a smaller breaking elongation although they had a larger tensile strength than bulk pure titanium. NTRODUCTON Various useful and important microsystems using thin films have been developed. To realize reliable and practical microsystems, characterization of the mechanical properties of thin films is important. However, little is known about the mechanical properties of small materials such as thin films because methods of testing and measuring their properties have not been established yet due to severe handling and measuring difficulties. n the case of tensile tests, which are most basic material tests, it is difficult to grip micro specimens and to measure minute elongation of the specimens. Therefore, the development of procedures for micro tensile tests is an important subject To establish such tests, the authors developed a novel tensile machine to measure the mechanical properties of small materials such as thin films, and proved that the tensile machine can measure their important mechanical properties such as tensile strength and breaking elongation [4, 51. The tensile machine can apply a load to small materials using a parallel spring-a mechanism that is free from friction--as a straight guide for the load. However, it was very difficult to hold a thin film in order to measure its mechanical properties using the above tensile machine [S. This paper describes the results of tensile tests using a new micrometer-based gripping device to hold thin film specimens [6]. Titanium thin films can easily be microfabricated by semiconductor manufacturing technology and are widely used in various types of microsystems such as strain gauges and thermal sensors. This paper describes the results of tensile tests on titanium thin films of.5,um thickness. Shape and dimensions SPECMENS Specimens were formed by magnetron sputtering. The specimens were titanium thin films of.5 ym thickness. The test section of these specimens was 3 ym in /97/$ EEE 43

2 , (.,,'=;;,.E;" Gauge mark (CrO) t : 5 nm Si frame.*.*.,....,., Y Thin film beam w:3 um deposited onto the test film. Specimens of 1. to 1.4 mm gauge length (length of the test section: 1.4 mm) were produced in order to examine the effect of gauge length on the measurements of mechanical properties. Figure 2 shows a micrograph of two gauge marks on a test film. Test thin film Ti:t=.5 um G. L : Gauge Length 1. to 1.4 mm t a) Poi yi m id e / Test thin film Ti Fig. 1. Titanium thin film specimens. 8 8 Fig. 3. Fabrication ~teps of specimens. Fabrication steps Fig. 2. Photograph of two gauge marks on a test film. width, and 1.4 mm in length. The stress concentration factor at the fillets of the specimens was about 1.5. Figure 1 shows the shape and dimensions of the specimens. The qxcimen was formed on a silicon substrate, comprising a test thin film and a silicon frame to protect the test film. The test film was connected to the silicon frame through four beams (width: 3 y m) of the identical thin film. The beams were cut off with a needle dter a specimen was mounted ou the tensile machine. To measure elongation, hvo gauge marks oc chromium oxide thiu films of 5 urn thickness were The main sequence of producing the specimens is shown below (see Figure 3). (a) Two silicon nitride thin films of.4 ym thickness were formed on both sides of a silicon substrate by chemical vapor deposition (CVD). n the next step, a polyimide film of 5 km thickness, to be used as a sacrificial layer, was coated on one side of the substrate by spin coating, and a titanium ted thin film was formed on the polyimide film by magnetron sputtering. After a chromium oxide thin film was evaporated onto the titanium test thin film, the two gauge marks were patterned. n the next step, the shape of the titanium test thin film was patterned by photolithography. 43 1

3 Double-field-of-view microscope \ \, w n s e g, Specimen Gauge marks Dividing mirror Mirror Fields of view Half mirror Optical fiber Thin plates Objective Stage / Movable grip Specimen Gripping device Fig. A Tensile Part of the back side of the silicon nitride thin film was patterned by photolithography, and then was removed by reactive ion etching (WE). n the next step, the silicon substrate from which the silicon nitride thin film had been removed was etched by a KOH solution from the back side. During this process, the silicon nitride thin films on both sides acted as etching stoppers. The front side was prevented from being etched by sealing the silicon wafer with an ring. (c)the silicon nitride thin films were removed from the back side by RE During this process, the polyimide film acted as an etching stopper. n the final step, the polyimide film was removed by oxygen plasma ashing. Tensile machine EXPERMENTAL Figure 4 shows an outline of the tensile machine. Specimens were attached to a movable grip and to a fixing side - gripping device. A parallel spring was made of four thin plates to serve as the straight guide mechanism for the movable grip. The movable grip and the straight guide mechanism were lifted over the surface plate of the tensile machine so that they were not affected by friction which would otherwise seriously impair the accuracy of the tensile tests. Load was applied by pulling (using a precision translation stage driven by a d.c. motor) one end of a steel belt, the other end of which was connected to the movable grip. A load cell with a capacity of.49 N was used to measure the load, which was the sum of the loads applied to the specimen and the parallel spring. The load applied to the specimen was calculated by subtracting from the measured load the load applied to the parallel spring calculated from the characteristics of the parallel spring measured in advance. Elongation measuring device The elongation was determined by measuring the relative displacement of the two gauge marks on the specimen. The measuring device consisted of a doublefield-of-view microscope (magnification: E), two CCD cameras (213 inches with 41 thousand pixels), an image synthesizer, and an image processor. The double-fieldof-view microscope divides the observed image by one objective into two with a dividing mirror and enlarges each image separately. This allowed us to enlarge and observe only the areas around the two marks in a specimen, so the microscope could continuously measure the elongation with a high resolution. Figure 5 shows a micrograph of two gauge marks observed by the double-field-of-view microscope. The relative displace- 432

4 ment of the two marks in the single field of view could be measured. However, the resolution of this method is insufficient because the elongation is far smaller than the gauge length. To assess the accuracy of this elongation measuring device, the displacements measured by it were compared with the displacements measured by a triangulation laser displacement sensor (repeatability: k.5 y m). The displacements measured by the two methods agreed very well, and it was verified that the measuring device has a measuring accuracy of 1,U m or better [4]. specimen on the tensile machine. n the first step, the silicon frame of the specimen was mounted on the movable grip by clamping it with a screw and a plate. n the next step, the specimen was set by holding the test thin film between two fingers on the fixing side - gripping device using a micrometer. n the final step, the four thin film beams connecting the test thin film to the silicon frame were cut off with a needle. RESULTS AND DSCUSSON By using specimens protected with silicon frames and a novel tensile machine, tensile tests of thin films were easily conducted. Furthermore, by holding a thin film specimen with a gripping device using a micrometer, it was easy to mount the specimen on the tensile machine. Tensile tests were conducted at room temperature in the atmosphere at a loading speed of.8 ym per second. The stress-strain diagrams of titanium thin films were measured continuously from elastic deformation to fracture. Fig. 5. Gauge marks observed by the double-field-ofview microscope. Scr Micrometer Fl Figure 7 shows a micrograph of a fractured titanium specimen. The photograph indicates fracture of the titanium thin films was due to brittlenesswith almost no plastic deformation. Figure 8 shows a measured stressstrain diagram of a titanium thin film, indicating that the material has a low ductility. Measured stress-strain diagrams correspond with the fact that the titanium specimens broke due to brittleness with almost no plastic deformation in Figure 7. Plate \ Q Specimen Movabley,,*a, \, nb \ Fingers Brittle fracture Fig. 6. Method of mounting the specimen. Method of mounting the specimen Fibwre 6 shows the simple method of mounting a Fig. 7. Photograph of a fractured Ti thin film specimen. 433

5 1. a 8 * Thickness :.5 btm Width : 3 pm Gauge length : 1.4 mm.4 v strain a (3 Q Fq. 8. Stress-strain diagram of a Ti thin film specimen. 1.o Gauge Length mm Fig. 1. Dependence of tensile strength on gauge length. 8 1.o Gauge Length mm Fig. 9. Dependence of Young's modulus on gauge length. Tensile tests were conducted on specimens of 1. to 1.4 mm gauge length (length of the test section: 1.4 mm) in order to examine the effect of gauge length on the measurements of mechanic a1 properties. Figure 9, Figure 1, and Figure 11 show the effect of gauge length on the measurements of Young's modulus, tensile strength, and breaking elongation respectively. The experimental results indicated that measurements of Young's modulus, tensile strength and breaking elongation were independent of gauge length for our range of measurements. The stress concentrations at fillets of the specimens (stress concentration factor: 1.5) appear to have little effect on the measurements of mechanical properties. 1.o Gauge Length mm Fig. 11. Dependence of breaking elongation on gauge length. The average value of Young's modulus, tensile strength, and breaking elongation is 96 GPa with a standard deviation of 12 GPa,.95 GPa with a standard deviation of.15 GPa, and 1.3% with a standard deviation of.4% respectively. The measurements showed that the titanium thin films had a higher strength but smaller plastic deformation than bulk 99.9% pure titanium (tensile strength:.235 GPa, breaking elongation: 54%) i 434

6 [7], though the causes for this have not yet been clarified. The microstructure of the titanium thin film appears to suppress plastic deformation like worked and hardened materials because its mechanical properties are similar to those of worked and hardened materials. CONCLUSON By using specimens protected with silicon frames and a novel tensile machine, tensile tests of thin films can easily be conducted. Furthermore, by holding the thin film specimens with a gripping device using a micrometer, it is much easier to mount the specimens on the tensile machine. The specimens were tested to determine the continuous stress-strain diagrams. Tensile tests were conducted on specimens of 1. to 1.4 mm gauge length in order to examine the effect of gauge length on the measurements of mechanical properties. Measurements of Young's modulus, tensile strength and breaking elongation were independent of gauge length for our range of measurements. The experimental results indicated that the titanium thin films had a higher strength but lower plastic deformation than bulk pure titanium. The authors will continue to study the effects of the chemical composition, process conditions, microstructures, and surface conditions of various thin films on their mechanical properties. REFERENCES [l] W. N. Sharpe, Jr., B. Yuan and R. Vaidyanathan, New test structures and techniques for measurement of mechanical properties of MEMS materials, Proc. SPES 1996 Symp. on Micromachining and Microfabrication, Microlithography and Metrology in Micromachining 11, Vol. 288, Austin, U.S.A., October 14-15, 1996, pp [Z] D. T. Read and J. C. Marshall, Measurements of fracture strength and Young's modulus of surfacemicromachined polysilicon, Proc. SPES 1996 Symp. on Micromachining and Microfabrication, Microlithography and Metrology in Micromachining 11, Vol. 288, Austin, U.S.A., October 14-15, 1996, pp [3] J. Dual and E Mazza, Microsample tensile test: Experiments and theory for the mechanical characterization of single crystal silicon microstructures, Proc. 5th nternational Conference on Micro Electro, Opto, Mechanical Systems and Components, Potsdam, Germany, September 17-19, 1996, pp [4] Ogawa, Y. shikawa and T. Kitahara, Measurements of stress-strain diagrams on micro materials by a developed tensile machine, Prepr. 73rd JSME Fall Annual Meeting, Vol., Fukuoka, Japan, September 11-13, 1995, pp [5] Y. shikawa, R Ogawa, T. Kitahara and N. Ooyama, Evaluation of micro motion mechanism, Proc. 1st nternational Micromachine Symp., Tokyo, Japan, November 1-2, 1995, pp [6] H. Ogawa, Y. shikawa, T. Kitahara and S. Kaneko, Measurements of stress-strain diagrams of thin films by a developed tensile machine, Proc. SPES 1996 Symp. on Micromachining and Microfabrication, Microlithography and Metrology in Micromachining 11, Vol. 288, Austin, U.S.A., October 14-15, 1996, pp [7] W. S. Lyman and D. R Wilson, Pure metals - Titanium (Ti), n: Metals Handbook, 9th edition, Vol. 2, Metals Park American Society for Metals, 1979, pp