Tensile Testing of Polycrystalline Silicon Thin Films Using Electrostatic

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1 Paper Tensile Testing of Polycrystalline Silicon Thin Films Using Electrostatic Force Grip Member Toshiyuki Tsuchiya (Toyota Central Labs., Inc.) Member Osamu Tabata (Ritsumeikan University) Jiro Sakata (Toyota Central R&D Labs., Inc.) Member Yasunori Taga (Toyota Central Labs., Inc.) Summary In this paper, a new tensile tester for thin films is presented. This tensile tester has a grip that fixes a thin film specimen using electrostatic force. The tester was constructed in a scanning electron microscope (SEM) chamber. Using this tester, the tensile strengths of polycrystalline silicon (poly-si) thin films have been measured. The tested part of the specimen is long, wide and thick. The fracture of the poly-si thin film was brittle. The mean tensile strength was GPa, depending on the length of the tested part. The size of the critical flaw that initiates fracture of the poly-si thin film is 28-47nm, rather small than the grain size of the poly-si thin film. Key words: mechanical properties, thin films, strength, tensile test, poly-si, electrostatic force. 1. Introduction Mechanical properties of thin films with a thickness of several microns as structural materials are matters of great concern to many engineers in micro sensors. Thus, measurements of mechanical properties such as Young's modulus, Poisson's ratio, and internal stress which are essential to designing micro sensors with suitable performance, are widely investigated. With the knowledge of these properties, various micro sensors including acceleration sensors(1) and pressure sensors(2) have already been fabricated. Recently, the development of these micro sensors has been changing from the research stage to the production stage. Under such circumstances, the mechanical strength of thin films has become indispensable to the engineers in order to assure the reliability of sensors. The mechanical strengths of the thin films have been measured with a membrane fracture test by applying pressure (bulge test)(3, 4) and a cantilever beam bending test(5,6). However, thin film will fracture at the edge of a membrane or a beam due to stress concentration, and the measured values are affected by the shape of the samples. Therefore, these techniques are not available as the standard techniques for measuring the exact values of the strength. To reveal the fracture mechanism of thin films and the relationship of the mechanical strength with microstructures of thin films such as grain size and surface roughness of thin films, the exact value of the mechanical strength is required. The promising way to measure the exact mechanical strength of thin films is a uni-axial tensile test. However, the tensile test of thin films is very difficult. Once a specimen is released from its substrate, it is too small to be manipulated and to fix to a tester by screwing them up to the grips of the tester. Koskinen et al. used an adhesive to fix the specimen to the grips(7). This is a easy way to fix the specimen to a grip, but it is difficult to release the fractured specimen and to clean up the probe for the next testing. It takes a long time to test many specimens. Therefore, we need a new grip system easy to fix a specimen and to release it after it fractures. In this paper we propose a new thin film tensile tester using the grip that fixes a specimen by electrostatic force. Electrostatic force is a weak force compared with the mechanical force and is not suitable for tensile testing for bulk materials, but is strong enough to fix a thin film. A specimen can be easily fixed to and released from the tester grip by applying and cutting off the electrical power supply. Therefore, a large number of specimens that are enough to perform statistical evaluation can be tested sequentially without any troublesome sample preparation. The tester was constructed in a scanning electron microscope (SEM) chamber and was applied to the measurement of the tensile strength of a polycrystalline silicon

2 Figure 1 Schematic drawing of tensile testing using electrostatic force grip. (poly-si) thin film that is the most frequently used material for micro sensors. The structure of the new tensile tester is explained first, the sample fabrication process and experimental results are then shown, and discussion is followed. 2. Tensile tester for thin films Figure 1 shows the concept of a tensile testing using an electrostatic force grip. One end of a specimen is fixed to a silicon wafer as a substrate. The other end is free from the substrate, and is fixed to a probe by electrostatic force. The probe is made of a conductive material covered with an insulating film. Figure 2 shows a procedure of the tensile test. First, the probe is closed to the free end of the specimen (Fig. 2a). While the substrate has the same electrical potential as the specimen, a voltage is applied between the specimen and the probe. The charge on their surfaces generates electrostatic force to fix each other (Fig. 2b). While applying the voltage, the tensile force is loaded to the specimen by moving the probe, until the specimen fractures (Fig. 2c). By applying the opposite pole voltage between the probe and the substrate, the charge remaining at the specimen generates repulsive force to release from the probe and attractive force to fix to the substrate. The free end of the fractured specimen is then removed (Fig. 2d). In this test, the specimen can be handled as fabricated on the substrate and can be fixed to the tester without touching the thin film. Electrostatic force to fix the specimen is calculated as follows: The tested part is wide, the friction force FF. The electrostatic force FH acting thick as shown in Fig. 1. We use a silicon wafer covered with a silicon nitride film (Si3N4) as a probe. To fix the free end of the specimen to the probe, the electrostatic force acting perpendicularly to two parallel electrodes (Fv) is used. Fv is calculated as follows: where S is the area of the free end of specimen, d is a gap between the probe and the free end, V is the applied voltage, Figure 2 Procedure of tensile test of thin film using electrostatic the permitivity constant When Fv is bigger than the weight of the specimen, it will stick to the probe: where g is the gravity and m, t and the mass, thickness and density of the specimen, respectively. If the specimen is poly-si thin film it will stick at d =1mm with V=100V. To fix the specimen without slipping to the probe while the tensile force is loaded, other forces parallel to the tensile stress is needed; the pure electrostatic force FH and parallel to two electrodes force. is where wg is the width of the free end, a gap and 442 T. IEE Japan, Vol. 116-E, No. 10, '96

3 Figure 3 Tensile tester for thin film. Figure 4 Fabricating process of specimen. a) depositing films, b) patterning poly-si and Al films, c) sacrificial etching to release poly-si film. dielectric constant between electrodes, respectively. When the specimen is fixed, d is equal to the thickness of Si3N4 as the insulating film is of Si3N4 8). The friction force FF is, where friction coefficient between the specimen and the probe. We assume 0.3. The sum of the two forces has to be bigger than the fracture tensile force FT, where w is the width of the tested part of the specimen (w Since the tensile strength be several GPa, the largest value of FT is about 0.1 N. The applied voltage V is 100V, considering the electrical breakdown strength of the insulating film (several NW/cm). Assuming that the free end of the specimen is square (S=wg2), the area of this electrode need to be more than In this case, the friction force FF is dominant to fix the specimen, and the electrostatic force FH that is acting parallel to the specimen is about 1/1000 of FF. Therefore the electrostatic force FH is much smaller than the fracture tensile force FT. If no friction force was produced, the width of the specimen would have to be 60mm. Figure 3 shows a setup of the tensile tester in a SEM chamber to observe the tensile testing procedure. The specimen placed at a precise stage is aligned with a probe at a probe stage and a SEM specimen stage on the SEM specimen stage. The precise stage is driven by a piezoelectric actuator, and apply tensile force to a specimen. Displacement of the precise stage and applied tensile force are monitored with loadcells. 3. Tensile testing of poly-si thin films This tester is applied to the measurement of the tensile strength for poly-si thin films. The poly-si specimens with their free end released from the substrate is fabricated with the micromachining process shown in Figure 4. LPCVD Si3N4 and PCVD USG (Undoped Silicate Glass; are deposited on a Si wafer as an insulating layer and a sacrificial layer, respectively. LPCVD amorphous silicon film is deposited at and annealed Figure 5 SEM micrograph of specimens at to crystallize. The grain size of the crystallized silicon film is about This poly-si thin film has tensile internal stress that is suitable for micro structures. The poly-si film is patterned into the shape of the specimen by the photolithography and the Reactive Ion Etching (RIE). An aluminum pad as an electrode is fabricated on the fixed end of the specimen. The sacrificial layer is removed by wet etching with HF. After the etching, the specimen is rinsed with distilled water and 2- propanol, and dipped with hot p-dichlorobenzene, then dried without sticking the specimen to the substrate.

4 Figure 6 SEM micrograph of tensile testing of thin film Figure 8 Relationship with size of specimen and long. The applied tensile force was linearly increased with the displacement of the precise stage until the specimen fractured. No offset tensile stress is observed at the start of the testing, and we assured that the electrostatic force FH have not affected the measured tensile stress. We can easily recognize whether the specimen slipped or not by this stress-displacement curve. When it slipped, the curve Figure 7 Result of tensile testing of poly-si thin film. Figure 5 shows SEM micrograph of the specimens. A number of specimens are fabricated on one substrate. The free ends measuring 1mm long and 0.2mm wide have many small etching holes to shorten the etching time. With these free ends, the voltage to fix the specimen is 60V. Both the free end and the fixed end are tapered off to the tested part not to fracture by stress concentration. Figure 6 shows a SEM micrograph of the poly-si thin films. Enough the specimen is V of the tensile testing voltage applied between and the probe to fix during the tensile testing for the specimen. It is rather high than that we have calculated. With this voltage, some specimens slipped while the tensile stress was increasing. There must be small particles in the contact area, and the friction force generated at an increasing gap is weaker than we have expected. Figure 7 shows a result of the tensile testing of the specimen that has the tested part wide and gradually decreased or repeated small drops with increasing the displacement. The tensile strength of this specimen was 2.6 GPa. The specimen showed no necking, and no plastic deformation was observed. The fracture surface was brittle. The specimens which are 30, 100, and long and wide have been tested. In Fig. 8, mean values, the largest and the smallest values, standard deviations of each specimen's tensile strength are plotted. The number of the tensile-tested specimens are shown below the plot. The specimen fractured in a random position within the tested part. The mean values of the tensile strength were GPa. These are comparable with the bending strength of single crystal silicon cantilever beams). The tensile strength decreased with increasing specimen length. The mean strength of the specimens was about 30% higher than that of specimens. The measured tensile strength varied widely for the same specimen size; the highest strength is about two times as high as the lowest strength in the same specimen length. The standard deviations were GPa that are about 20% of the mean strength. These values are relatively high to the mean strength. This scattering of the 444 T. IEE Japan, Vol. 116-E, No. 10, '96

5 measured tensile strength also decreased with increasing the length of the specimen. The standard deviation of the specimens was 25% lower than that of 300- specimens. 4. Discussion 4.1 Tensile tester using electrostatic force grip The maximum thickness of the specimen applicable to this tester is determined by the area of the available free end of the specimen and the friction coefficient between the specimen and the probe. If the friction coefficient is as same as our result, the maximum thickness will be about the largest area of the free end that we can easily fabricate is The electrostatic force grip is applicable to any other materials including insulating materials. For insulating materials, we can fix the specimen by arranging two electrodes on a probe or making an electrode on the free end of the specimen. 4.2 Tensile strength of poly-si thin films The tensile strength of poly-si has a size effect. A shorter specimen has a higher mean strength, and has a larger scattering of strength. These are typical phenomena with a strength of the brittle material. The brittle material fractures on the weakest point in the specimen, where the tensile stress is applied. The weakest point corresponds to the point that has the largest flaw exists, while many flaws are distributed along the specimen. The number of the flaws increases with increasing the size of specimens. Since the size of the flaws in specimen is distributed, size of the largest flaw in a specimen increases and the scattering of the largest flaw size among specimens decreases with increasing the specimen size. As a result, the tensile strength lowers and the standard deviation becomes small with its volume. Therefore, the existence of size effect of the poly-si tensile testing indicates that the fracture of poly-si thin film is initiated at the flaw within the tested part of the specimen. The fracture behavior of the poly-si thin film shown in Fig. 7 indicates that the poly-si is a brittle material. Since it fractures in a random position within the tested part, the stress concentration the at the ends of the tested part does not affect the tensile strength. Therefore, the measured values of this tensile testing is the exact tensile strength of the poly-si thin films. The measured mean tensile strength is 6-8% of the theoretical strength of single crystal silicon(8). With these values, the size of the flaw where fracture initiated, can be calculated as 28-47nm using Griffith's equation, that is smaller than the grain size of this film. has Since most bulk ceramics materials have a weak grain boundary where fracture initiates, the critical flaw size is similar to its grain size. However, the calculated flaw size of the poly-si thin film is rather small than the grain size. From this results, it can be concluded that poly-si grain boundary is not too weak to fracture and not the critical flaw. Therefore, other flaws such as surface roughness should be considered as the initiation of poly-si thin film fracture. 5. Conclusion A new tensile tester for thin films has been presented. This tester has a grip that fixes the thin film specimen easily using electrostatic force. The tensile strength of poly-si films has been measured using this tester, and tensile strengths of GPa were obtained, depending on its length of the tested part of the specimens. Since the size of the critical flaw that initiates fracture of the poly-si thin film is rather small than the grain size, the fracture origins are not the grain boundaries but other flaws such as surface roughness. In order to reveal the fracture origin of the poly-si film, the size effect of the tensile strength, and the relationship between the strength and the process conditions will be clarified in the next step. (Received Mar. 29, 1996 and revised Aug. 5, 1996) REFERENCES (1) T. A. Core, W. K. Tsang, and S. J. Sherman, "Fabrication technology for an integrated surface micromachined sensor", Solid State Technology, pp , (2) K. Shimaoka, O. Tabata and S. Sugiyama, "Micro- Diaphragm Pressure Sensor using Polysilicon Sacrificial Layer Etch-Stop Technique", Tech. Digest of the 7th Int. Conf. on Solid-State Sensors and Actuators, Yokohama, Japan, June 7-10, 1993, pp (3) J. W. Beams, in C. A. Neugebauer, J. D. Newkirk and D. A. Vermilyea(eds.), The Structure and Properties of Thin Films, Wiley, New York, p. 183, (4) A. J. Griffin, Jr., F. R. Brotzen, and C. F. Dunn, "Mechanical properties and microstructures of Al- 1%Si thin film metallizations", Thin Solid Films, vol. 150, pp , (5) S. Johanson, Schweitz, L. Tenerz, and J. Tiren, "Fracture testing of silicon microelements in situ in a scanning electron microscope", J. Appl. Phys., vol. 63, pp , 1988.

6 (6) T. P. Weihs, S. Hong, J. C. Bravman, and W. D. Nix, "Mechanical deflection of cantilever microbeams: A new technique for testing the mechanical properties of thin films", J. Mater. Res., vol. 3, pp , (7) J. Koskinen, J. E. Steinwall, R. Soave, and H. H. Johnson, "Microtensile testing of free-standing polysilicon fibers of various grain sizes," J. Micromech. Microeng., vol. 3, pp , (8) A. Kelly, Strong Solids, Clarendon Press, Oxford, Toshiyuki Tsuchiya (Member) received the M. S. degree from the University of Tokyo in Then, he joined Toyota Central R. & D. Labs., Inc., where he works on silicon micromachining and thin film mechanical properties evaluation. Osamu Tabata (Member) received the M. S. degree and the Ph.D. degree from Nagoya Institute of Technology in 1981 and 1993, respectively. Since 1981, he has been with Toyota Central R & D Labs., Inc.. In 1996 he joined the Department of Mechanical Engineering, Retumeikan University. He works on silicon micrimachining, thin film mechanical property evaluation, integrated silicon sensors and MEMS. Jiro Sakata received the M. S. degree from Nagoya University in 1978 and the Ph.D. degree from the University of Tokyo in Since 1978, he has been with Toyota Central R & D Labs., Inc.. Now he works on thin films technology. Yasunori and interface Taga (Member) joined Toyota Central R & D Labs., Inc. in 1970 after graduation from Nagoya Institute of Technology. He received the Ph.D. degree from Osaka University in He became a manager of Electronics Device Division, where he works on thin film and surface Physics.