Proceedings of IMECE ASME International Mechanical Engineering Congress and Exposition November 5-10, 2006, Chicago, Illinois USA

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1 Proceedings of IMECE ASME International Mechanical Engineering Congress and Exposition November 5-10, 2006, Chicago, Illinois USA IMECE RAPID SILICON-TO-STEEL BONDING USING INDUCTIVE HEATING Brian D. Sosnowchik * Department of Mechanical Engineering Berkeley Sensor & Actuator Center 497 Cory Hall, Berkeley, CA Liwei Lin Department of Mechanical Engineering Berkeley Sensor & Actuator Center 497 Cory Hall, Berkeley, CA Albert P. Pisano Department of Mechanical Engineering Berkeley Sensor & Actuator Center 497 Cory Hall, Berkeley, CA ABSTRACT In this work, we present a rapid, low temperature process for the bonding of silicon to steel through the use of inductive heating for MEMS sensor applications. The bonding process takes as short as three seconds with a maximum bonding temperature as low as 230 o C at the steel surface. The bonding strength is strong, and causes minimal damage to steel. The process has also been shown to work using leaded and leadfree bonding solder with minimal surface preparation to the steel. Four characterization experiments tensile and compressive 4-point bend, axial extension, and fatigue tests have been performed to validate the bonding process and materials. As such, this work illustrates the promise of applying inductive heating for the rapid silicon bonding to steel components for MEMS sensing applications. INTRODUCTION For the past several decades, micorelectromechanical systems (MEMS) have garnered the interest of a myriad of research groups. Universities have well-established and dedicated centers devoted to the study, while several industrial companies have been founded with core technologies involving both simple and complex MEMS devices. MEMS technologies have allowed for the development of sensors with previously unattainable sensitivity, size, and system-level integration. However, in order for the plethora of developed MEMS devices to be implemented in large-scale systems or * Contact Author bdsosnow@me.berkeley.edu environments, a considerable investigation must be undertaken into how the device will be packaged and placed in the area of deployment. Encapsulation is traditionally entailed in such an investigation, but how the device will remain bonded in the area that it is sensing and how it will survive in the unique environments enabled by MEMS are becoming greater concerns. As the technology advances and key sensor parameters improve, it becomes necessary to reexamine bonding technologies for the rapid deployment of siliconbased sensors into large systems. For example, steel is a very important and commonly used material in a wealth of industries. To manufacturably bond a MEMS sensor to steel, however, several design parameters should be considered. First, the bonding process must be rapid, with a reliable, repeatable bond occurring in a short period of time. Second, the bonding process must not do damage to the steel. For heat treated steel, the temperature should remain low enough to prevent martensite transformation. Third, the bond must be able to survive in oil and lubricant filled environments. Finally, for the bonding process to be fast and low-cost, it should employ simple equipment. Given these constraints, many previously reported bonding methods would not be suitable for large-scale, robust deployment. For example, one method of bonding silicon to steel would be through the use of an epoxy bond. It is easy to implement, but epoxy bonds will not survive in harsh environments and will have undesirable mechanical characteristics (creep, fatigue, etc). Diffusion bonding is

2 another method of bonding silicon to steel [1], but the bonding process necessitates high temperatures. Lasers may be used to locally bond silicon to steel in a low-temperature process [2], but the process requires a complex, and expensive piece of lasing equipment. Finally, microheaters could be used to form a bond [3, 4], but additional processing to the MEMS chip is needed. In the past, researchers have demonstrated the eutectic bonding of silicon to steel using a rapid thermal annealing method [5]. Heat generation in the steel using rapid thermal annealing occurs through the use of external infrared heating elements. Thus, such a bulk heating method necessitates longer bonding times. Conversely, inductive heating represents a different method of heat generation. Since the heat generation occurs locally within the material, heating can occur rapidly and may be isolated to a single, small location on the steel. Inductive heating is a fast, clean, and inexpensive process. Figure 1 shows the fundamental principle of inductive heating. By initiating a large, alternating current within an induction coil, a substantial alternating magnetic field is generated. This alternating magnetic field propagates a short distance into metals, producing eddy currents which cause Joule heating [6]. Because of the low resistivity of metals, the high magnetic permeability constant of steel, and the high applied frequency of the inductive heater, the Joule heating is localized to within the skin depth, δ, of the steel, which is on the order of microns and by definition, where 84% of the incident power is transferred to the steel. Moreover, because the majority of the heat generation is localized to the within the skin depth, the temperature is theoretically the highest at the surface of the steel where the bond is formed. Inductive Heating Overview Depth, x Induction Coil Silicon Power Density, I Steel Fig. 1: Fundamentals of inductive heating. Skin Depth, δ, ρ δ= πµ r µ o F ρ Resistivity F Applied Frequency µ r Relative Magnetic Permeability Power Density, I, x δ I= I 0 e Incident power density is transferred exponentially vs. depth into steel Fig. 2: Process flow for silicon to steel bonding using inductive heating. EXPERIMENTAL SETUP Figure 2 illustrates the process flow for silicon to steel bonding using inductive heating. Silicon will not form a natural bond with steel at low temperatures. Therefore, several material systems compatible with both silicon and steel were used in this work involving low-temperature eutectic solders and metallic adhesion layers. Lowtemperature eutectic solders were selected because of their ability to bond nicely with steel and to remain within the aforementioned design constraints. The two solders used for this work were 96.5Sn/3Ag/0.5Cu lead-free solder paste (Hi- Performance Lead-Free No-Clean Solder Paste, S3X58-M405, Koki Company Ltd.) and electroplated 63Sn/37Pb (Techni Solder MatteNF 820 HS 60/40, Technic Inc.). Metallic adhesion layers deposited onto the surface of silicon wafers (p-type, (100), 10-50Ω-cm) were used to promote the intermetallic bonding of the silicon to the steel, including: Silver : 500Å chromium / 7000Å silver thermally evaporated Gold : 200Å chromium / 1000Å gold thermally evaporated Nickel : 500Å chromium / 5000Å copper thermally evaporated / 1.5µm electroplated nickel (Techni Nickel Sulfamate, Technic Inc.) / Si

3 1.25µm electroplated 63Sn/37Pb eutectic solder (Techni Solder MatteNF 820 HS 60/40, Technic Inc.) The silicon wafers were diced into 5x5mm 2 test specimens, and bonding was performed on 1095 spring steel parallels that were 6in long, 1/32in thick and 7/8in to 1-3/16in wide. The steel parallels were roughened with 150 grade sandpaper to remove the thick oxide present on the parallel surface and cleaned with acetone. Two bonding methods were tested using lead-free solder and electroplated 63Sn/37Pb solder, respectively. For the first method, about 1mm 3 in volume of lead-free solder paste was applied manually to the center of the steel sample, which resulted in a post-bonding thickness targeting approximately 40µm. A silicon test specimen, from the aforementioned silicon chips with one of the three metallic adhesion layers, was then placed onto the solder with the adhesion layer contacting the solder paste. This system was then placed 9mm under the induction coil. The inductive heating system was then initiated at a power ranging from W for 3-5 seconds. During this time, the steel is rapidly heated which melts the solder and forms the bond. For the second method, a 40µm-thick, 63Sn/37Pb solder layer was deposited on a layer of nickel (the third type metallic adhesion layer in the previous section). These Solder-On-Chip test specimens (SOC) were then placed onto the steel sample, and bonded with a pressure of 1800Pa in the aforementioned method. or delaminate. gauges were used to obtain a better understanding of failure quantitatively. Second, foil strain gauges were used to assess the level of strain transmission through the chip, which is important for practical MEMS sensor applications. Four different tests were performed on the bonded samples using an Instron tensile testing apparatus. They were as follows: Tensile 4-Point Bend Test: Load was slowly applied in a 4-point bend configuration to bend the bonded samples in tension. Compressive 4-Point Bend Test: Load was slowly applied in a 4-point bend configuration to bend the bonded samples in compression. Axial Extension Test: Load was slowly applied along the length of the steel. Fatigue Test: Steel sample was cycled with load corresponding to 1000µε at the steel surface in a tensile 4-point bend configuration. During each test, the strain measured at the silicon surface and the load data were recorded. In each case, theoretical values of the stress and strain at the steel surface were calculated, whereby the stress for the case of bending was obtained with equation (1) and the stress for the case of axial extension was obtained with equation (2) as follows σ = PL bend 2 2wt (1) P σ axial= wt (2) Foil Gage Fig. 3: Foil strain gages were affixed to the surface of the bonded silicon to monitor the strain vs. applied load. Foil strain gages (gage length 1.57mm, gage width 3.05mm) were subsequently adhered with Vishay M-Bond 610 to the surface of the bonded silicon samples as shown in Figure 3. This was done for two reasons. First, during initial testing, the chips were observed qualitatively to either fracture where P is the applied load, L is the effective length of the 4- point bend setup, w is the width of the steel, and t is the thickness of the steel. RESULTS The temperature of the steel is an important parameter to monitor during the bonding process to ensure that the steel remains below the 230 o C temperature limit (typically considered safe to steel treatment for a short time) and also to help understand the thermal gradients that occur during heating. This was done using two methods. First, temperature indicating paints were used to establish an initial understanding of the temperature gradients that develop in the steel and also to determine an acceptable position of the steel with respect to the induction coil that allows the bonding to occur below the aforementioned temperature limit. Once the appropriate bonding parameters had been determined, an infrared camera was used to monitor the temperature during the bonding process. Figure 4 illustrates thermal IR imaging of a silicon chip being bonded to steel near the edge of the steel. During these

4 initial tests, the temperature threshold of 230 o C was exceeded, with the temperature on the steel reaching its highest point at the edge. Because of this initial imaging, subsequent bonding tests employed the use of a 400µm-thick copper shim placed on both sides of the steel at a distance of about 0.25in from the steel edge to serve as a heat-sink. Because of the shadowing effects of the inductor coil, infrared thermal imaging was not possible for this setup. However, temperature indicating paint near the edge of the bonded silicon samples verified that the temperature remained below 260 o C. that the silicon layer has delaminated. Finally, the output of some samples had a combination of delamination and fracturing, as illustrated by the chip with the silver adhesion layer. Silicon Chip Time = 2 sec 5mm Steel Perimeter 5mm Silicon Failed At 1516µε, 1432µε, and 1204µε Fig. 5: Output of the tensile four-point bend test with 550µm specimen bonded with lead-free solder. Time = 2.5 sec 5mm Table 1: Tensile 4-Point Bend Test Results Peak vs. Stress Slope 550µm / Au 1516µε Fracture 3.59µε/MPa 550µm / Ag 1432µε Delamination 2.88µε/MPa 550µm / Ni 1204µε Delamination 3.03µε/MPa 550µm SOC 891µε Fracture 3.78µε/MPa 300µm / Ni 1168µε Fracture 4.40µε/MPa 100µm / Ni 2127µε Fracture 5.09µε/MPa Time = 3 sec Fig. 4: Oblique angle infrared imaging of bonding at the steel edge. The perimeter of the steel is shown by the dashed line. The steel edge heats rapidly during the inductive heating process. To mitigate this occurrence, copper heat sink shims were placed under the edges of the steel. Figure 5 illustrates the results of the tensile 4-point bend test performed on a 550µm-thick silicon specimen bonded with lead-free solder. The stress vs. time measurement data are obtained by using equations (1) and (2) with the load vs. time data obtained from the Instron tensile testing machine. As the load on the steel is increased, the output of the sample with the gold adhesion layer increases abruptly at 1516µε, indicating that the silicon has fractured along the thickness of the chip. Visual inspection of the sample after the test was used to confirm this. Conversely, the output of the sample with the nickel layer decreases abruptly at 1432µε, indicating Table 1 summarizes the results of the tensile 4-point bend tests. In general, the majority of the specimens survive above 1000µε at the silicon surface, with the majority failing by means of the silicon fracturing along the chip thickness. Additionally, the last column is the result of strain/stress slope, which is defined as the change in the measured strain at the silicon surface over the change in the stress on the steel surface. It is observed that this value increases as the chip thickness decreases and it indicates that the amount of strain transfer increases as the chip thickness decreases under similar applied stress conditions. Figure 6 illustrates the percent of strain transfer through the silicon chip with respect to applied stress at the steel surface, which was defined as the ratio of the strain measured at the silicon surface to the strain imposed on the steel by the Instron tensile testing machine. For each specimen, the strain transfer shows a slight downward trend for increased stress magnitudes, but this trend downward is observed at larger stresses for the thinner chips.

5 Fig. 6: Percent strain transfer vs. applied stress for the tensile 4- point bend tests. The magnitude of the transferred strain increases as the chip thickness decreases. Figure 7 illustrates the results for the compressive 4-point bend test for the 550µm specimen bonded with lead-free solder. For these samples, the output remained linear for all samples with a slight deviation from linearity for the specimen with gold and silver adhesion layers observed at higher loads. Fig. 8: Percent strain transfer vs. applied stress for the compressive 4-point bend tests. The magnitude of the transferred strain increases as the chip thickness decreases. Table 2 summarizes the results obtained for the compressive 4-point bend tests. The specimens sustained substantially more strain, with each sample measuring approximately 2000µε or more on top of the silicon surface. Additionally, similar to the tensile 4-point bend test, the slopes of the linear portion of the measured strain vs. applied stress curve increase with decreasing chip thickness. Figure 8 shows the percent strain transfer through the silicon chip vs. applied stress at the steel surface for the compressive 4-point bend test. It is observed that as the stress increases, the decay of the strain transfer curve is greater for the thinner chip samples. Figure 9 illustrates the results of the axial extension tests for the 300µm and 100µm silicon samples bonded with leadfree solder. For these samples, the slopes of the measured strain at the silicon surface vs. applied stress were linear, with slight deviations from linearity at larger stresses attributed to bond layer yielding. Fig. 7: Measured strain at the silicon surface vs. stress in the steel of the compressive 4-point bend test with 550µm specimen bonded with lead-free solder. Table 2: Compressive 4-Point Bend Test Results Peak vs. Stress Slope 550µm / Au -3075µε Did Not Fail 3.21µε/MPa 550µm / Ag -2474µε Did Not Fail 2.67µε/MPa 550µm / Ni -2067µε Fractured 3.31µε/MPa 550µm SOC -3430µε Fractured 3.43µε/MPa 300µm / Ni -2412µε Did Not Fail 3.59µε/MPa 100µm / Ni -1998µε Yielded 4.75µε/MPa Fig. 9: Measured strain at the silicon surface vs. applied stress for the 300µm- and 100µm-thick specimen bonded with lead-free solder using a nickel adhesion layer.

6 Table 3: Axial Extension Test Results Peak vs. Stress Slope 550µm / Au 135µε Yielded 0.39µε/MPa 550µm / Ag 119µε Yielded 0.32µε/MPa 550µm / Ni 149µε Yielded 0.45µε/MPa 300µm / Ni 929µε Yielded 1.59µε/MPa 100µm / Ni 2654µε Fracture 4.00µε/MPa Amplitude Zero- Load Output Table 3 summarizes the results of the axial extension tests. For the 550µm-thick specimen, the peak strains observed before yielding began were quite low, with none surpassing a silicon strain of 150µε. However, as the chip thickness decreased, the peak strain increased to 929µε for the 300µm-thick specimen, and 2654µε for the 100µm-thick specimen. As such, the rigidity of thicker silicon chips could play a significant role in the strain transfer through the chip and yielding of the bond layer for this type of loading. Figure 10 shows the percent strain transfer through the silicon chip vs. applied stress for the axial extension tests, illustrating the noticeable increase in the strain transfer with decrease in chip thickness. Fig. 10: Percent strain transfer vs. applied stress for the axial extension tests. Finally, Figure 11 shows the results of the 1000-cycle tensile 4-point bend fatigue test of the 100µm-thick specimen bonded with lead-free solder cycled at approximately 1Hz. The peak strain measured at the surface of the silicon was greater than 1000µε, which is consistent with the strain transfer result for the tensile 4-point bend test for the 100µmthick specimen. Furthermore, the zero-load output, corresponding to the strain measured at the silicon surface for the case of no loading, does not change, indicating that the bonding layer under the silicon has yielded minimally. Fig. 11: Measured strain at the silicon surface vs. data point for the 1000-cycle tensile 4-point bend fatigue test of 100µm-thick specimen bonded with lead-free solder using a nickel adhesion layer. The pre-load strain output represents the measured strain from the weight of the upper portion of the 4-point bending apparatus. Table 4 summarizes the results of the fatigue tests. For the 550µm-thick specimens, the values of the zero-load strain output after cycling fell between -100µε and -200µε, which arose from the yielding of the bonding layer. However, thinning the test samples resulted in a mitigation of the bond layer yielding. The 300µm-thick specimen had a -12µε zeroload strain output, while the 100µm-thick specimen had ostensibly no bond layer yielding. Table 4: Fatigue Test Results Pre-Load Output Amplitude Zero-Load Output 550µm / Au 745µε Yielded -194µε 550µm / Ag 690µε Yielded -110µε 550µm / Ni 722µε Yielded -153µε 300µm / Ni 861µε Slight Yield -12µε 100µm / Ni 951µε Did Not Fail None DISCUSSION In the above work, we have successfully demonstrated the use of inductive heating for the eutectic bonding of silicon to steel. The time required for bonding ranging from 3-5 seconds, which is relatively fast compared to other bonding processes. Additionally, through hardness testing, we observed minimal damage to the heat treatment of Rockwell C 62 hardness steel. The equipment is simple, clean, and inexpensive, and little surface treatment is needed for this bonding process. However, the process is not without its intricacies. Since the energy for bonding comes from the surface heat generation of an inductively-heated workpiece, the fundamentals of inductive heating become the primary limitations of the

7 bonding process. For example, the position of the steel with respect to the induction coil plays a significant role in the heat generation process. For our test setup, increasing the coilworkpiece distance by just a few millimeters can have a significant impact on the time required for bonding. Likewise, the geometry of the coil and the workpiece and the inductive coupling between the two also play a role in the heating process. Because of this, it is necessary to identify a set of bonding parameters that allow the process to remain within certain time and temperature ranges. Although no data were provided on the yield strength of this particular alloy, most lead-free solders will yield at stresses of 100MPa and less. Yet, according to the output of the strain transfer, the yielding of some of the 300µm- and 550µm-thick specimen did not begin until stresses well beyond this threshold were obtained. Because of this observation, the rigid silicon sample for these cases might have prevented the yielding of the bonding layer by providing a certain level of material system reinforcement. One possible application for this bonding process is for MEMS strain sensors [7], where the amount of strain transfer through the chip will dictate the sensitivity of the device. The 550µm-thick specimen are undesirable, since the percent of strain transferred through the chip is small for the axial extension tests, and the fatigue tests resulted in bond layer yielding. Conversely, for the tensile 4-point bend test, axial extension test, and fatigue test, the thinner chips were better for this application, resulting in greater strain transfer. However, simply thinning the chip as much as possible may not be ideal, as evident from the results in compression, where the percent strain transfer in the 100µm-thick specimen dropped much more rapidly than the thicker chips. As such, the thickness of the bonded chip plays a significant role in the performance of the bonding layer and should be carefully considered when selecting the deployed system or environment. CONCLUSION In summary, a method of rapidly bonding silicon to steel using inductive heating has been developed. The method can bond silicon specimen to steel using low-temperature eutectic solders and adhesion layers of evaporated gold, evaporated silver, or electroplated nickel. The bonding method was also found to do minimal damage to heat treated steel. Tests were performed to monitor the condition of the bond and to assess the amount of strain transferred through the silicon specimen. The results of these tests show that with chip thicknesses of 300µm and 100µm, the bonding layer is able to sustain several different methods of loading. Further tests are needed to obtain a greater understanding of the bond properties, but these initial results demonstrate a potentially manufacturable solution for the large-scale bonding of silicon to steel for sensor applications. ACKNOWLEDGEMENTS Funding for this work was supported by army research office (ARO) grant DAAD The authors would like to acknowledge the help of Prof. Oliver O Reilly, Prof. Carlos Fernandez-Pello as well as Dr. Anand Jog, Mr. Robert Azevedo, and Dr. Andrew Cao for valuable discussions. REFERENCES [1] Polanco, R., De Pablos, A., Miranzo, P. and Osendi, M. I., 2004, "Metal-Ceramic Interfaces: Joining Silicon Nitride- Stainless Steel", Applied Surface Science, 238 pp [2] Luo, C. and Lin, L., 2002, "The Application of Nanosecond-Pulsed Laser Welding Technology in MEMS Packaging With a Shadow Mask", Sensors and Actuators A, pp [3] Cheng, Y. T., Lin, L. and Najafi, K., 2000, "Localized Silicon Fusion and Eutectic Bonding for MEMS Fabrication and Packaging", Journal of Microelectromechanical Systems, 9 (1), pp [4] Lin, L., Cheng, Y. T. and Najafi, K., 1998, "Formation of Silicon-Gold Eutectic Bond Using Localized Heating Method", Japanese Journal of Applied Physics, 37 pp. L1412- L1414. [5] Sosnowchik, B. D., Azevedo, R. G., Cao, A., Lin, L. and Pisano, A. P., 2005, "Silicon-to-Steel Bonding Using Rapid Thermal Annealing", IEEE Transactions on Advanced Packaging, 28 (4), pp [6] Rudnev, V., Loveless, D., Raymond, C. and Black, M., 2003, Handbook of Induction Heating, Marcel Dekker, Inc., New York. [7] Wojciechowski, K. E., Boser, B. E. and Pisano, A. P., 2004, "A MEMS Resonant Sensor Operated in Air", 17th IEEE International Conference on Micro Electro Mechanical Systems, pp