TSV-Based Quartz Crystal Resonator Using 3D Integration and Si Packaging Technologies

Jian-Yu Shih1,Yen-Chi Chen2, Cheng-Hao Chiang1, Chih-Hung Chiu2, Yu- Chen Hu1, Chung-Lun Lo2, Chi-Chung Chang2, and Kuan-Neng Chen1*

1Department of Electronics Engineering, National Chiao Tung University, Hsinchu 300, Taiwan

2TXC Corporation, Taoyuan, Taiwan *Tel: 886-3-5712121 ext. 31558, Fax: 886-3-5724361, Email: knchen@mail.nctu.edu.tw

Abstract

In this paper, one novel packaging approach for crystal resonator with the quartz crystal is demonstrated, developed and characterized. The proposed crystal resonator of the novel package adopts several 3-D core technologies, such as Cu TSVs, thin-film Cu/Sn eutectic bonding, and wafer thinning. A 1210 crystal resonator with quartz blank mount is successfully developed with Cu TSVs that enables the electrical connection of the signal electrode on the quartz blank. The quartz blank is protected by the simultaneous formation of strength reinforcement and excellent sealing capacity of thin-film Cu/Sn eutectic bonding. With the well fabrication and the enhanced structure, excellent leakage results in the MIL-STD-883 hermetic test (8.5*10-9 ~1*10-8 Pa m3/sec by He leak detector) show the crystal resonator using 3D integration has the great performance and manufacturability to replace the conventional metal or ceramic enclosures for the next generation products.

Introduction In recent years, with the development of wireless

communication, the high performance, small form factor and low cost microelectronic products become significant to meet the need of market. For packaging, multi-chips integration is the trend of the present semiconductor industries [1-3]. 3-D IC technologies enable heterogeneous integration by the vertical stacking and TSV interconnection. As the result, packaging by using 3D technologies can offer a smaller area solution with lower power consumption [4-5].

Quartz crystal device requires specialized packaging with the protective cavity over the active area of the quartz blank in order to avoid mechanical attack and mass loading from moisture or other contamination. Conventional quartz crystal devices are packaged in metal or ceramic vacuum enclosures [6]. The challenges of quartz crystal devices include high material and manufacturing cost, large size, and difficulty of scaling down. However, high performance, small form factor, and low cost have become basic requirements in the crystal resonator market. Since 1612 (1.6 mm × 2.0 mm) crystal resonator is the current standard product, how to achieve smaller 1210 (1.2 mm × 1.0 mm) crystal resonator in the next three years has been the great challenge [7-10].

Since three-dimensional integration offers a solution for electronic products with advantages mentioned above and heterogeneous integration, in this research we demonstrate the application of key technologies in 3D integration for crystal package process. The 1210 quartz crystal unit package was

achieved by the thin-film Cu/Sn eutectic bonding, silicon wafer thinning process, and Cu TSV fabrication. This structure demonstrates the smaller form factor of future electronic products with better reliability and electrical performance. TSV formation was achieved by Cu bottom up method without voids. The crystal blank and the quartz crystal device were protected by the cavity of bonded Si-cap and TSV-based Si substrate wafers, which were eutectic metal bonded. The hermetic and reliability tests with thermal flow were performed to evaluate the quality of these crystal resonators. With the qualification of thermal aging and reliability tests, this quartz crystal resonator using 3D integration shows robust and no leakage path for future applications . The complete study of structure design, process condition, electrical characterization and reliability of the novel crystal resonator by using 3D integration scheme are presented to offer the solution and investigated for future crystal resonator products.

Crystal Unit Package Design and Simulation Figure l shows the schematic cross section diagram of a

1210 (1.2x1.0 mm2) crystal unit package. The Si-cap in the top thinned wafer is bonded to bottom Cu TSV wafer by Cu/Sn eutectic bonding.

Figure 1. Schematic of the cross-sectional view of crystal unit package structure with crystal quartz.

The electrical performance and reliability are the most important topics for package structure design. The maximum bending stress in the 1210 crystal unit package was simulated by COMSOL Multi-physics tool to evaluate the locations of possible damage caused by following processes and the proper thickness of top capping wafer though the temperature variation. The material characteristics for the reliability investigation of structure are as shown in Table 1. The best condition obtained from the above simulation results was then applied to the 1210 crystal unit package.

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Table 1. Material characteristics of crystal unit package.

Figure 2. Thermal stress distribution and simulation results of bending for different package structures with TSV and different thickness of Si-cap.

Figure 2 presents the simulation results of the maximum bending stress for different package structures and thickness of Si-cap by COMSOL Multi-physics tool. The bending stress by reducing the thickness of the 1210 crystal unit package structure (thickness of 400 to 200 μm; cavity depth of 180 μm) under the linear temperature rising from 300 to 700 K is shown in Figure 3.

Figure 3. The bending stress for package structures with different thickness of Si-cap.

From the simulation results, the maximum bending stress always occurs at the four corners of inner cavity and decreases with increasing thickness. Since the maximum 1.46 MPa bending stress is still below the plastic deformation stress, which may crack silicon substrate and inside devices, this design should be robust for the mechanical point of view.

The simulation results can provide good database for package design. In addition, the optimized structure for thermal stress and electrical characteristics was then designed and fabricated for silicon package.

Process Integration of Si-Cap Unit and Fabrication of Cu TSV

The process flow of the 1210 crystal unit package by using 3D integration and Si packaging technologies is shown in Figure 4. The cavity structure for Si-cap wafer was thinned and then fabricated by ICP DRIE method to form straight and smooth surface of cavity structure (cavity depth of 180 μm). The optimized ICP technique was demonstrated successfully and the dry etching recipes for the formation of cavity structure at Si-cap wafer by using different hard-mask materials were investigated. However, during the etching process, the particles of hard mask were knocked off due to the ion bombardment, which may lead to the micro-masking issue.

Figure 4. The process and integration flow of crystal unit package.

Therefore, we investigated the recipes of KOH etching to remove micro-masking on the bottom of cavity. In this research, different KOH recipes were evaluated to eliminate micro-masking problem. Table 2 and Figure 5 show the results, schematic diagram, and corresponding SEM images of silicon cavity etching with different hard mask materials and the removal of micro-masking by using different KOH etching parameters.

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Table 2. Results of micro-masking removal by different KOH etching parameters.

Figure 5. The profiles of silicon cavity with different hard mask materials before and after the micro-masking etching.

For replacing the metal lid of conventional crystal package of cap cavity, the pattern of eutectic bonding metal layer was developed on Si cap wafer by lift-off process. The 1.5 µm-thick Cu/Sn metal layer was formed on Si-cap wafer, as shown in Figure 6. Finally, a 200 µm-deep cavity with 100 µm-thick cap was successfully fabricated to replace the conventional metal lid or ceramic vacuum enclosures for cap cavity in crystal package.

Figure 6. (a)The etching profile of cavity; (b) Cu/Sn metal layer on the Si-Cap wafer by SEM.

Figure 7. (a) The process flow of bottom-up filled Cu TSV; (b) the TSV oxide liner profile; (c) the ECD Cu TSV profile.

The 100 µm-wide and 250 µm-deep Cu TSV was fabricated to connect the electrode and the crystal blank. Figure 7 shows the schematic of bottom-up filled Cu TSV by applying DRIE TSV, oxidation, wafer thinning, Cu seed layer deposition, and Cu electroplating. To ensure the fully hermetic sealing requirement of crystal unit package, the adhesion layer between TSV and Si substrate plays an important role. By using the bottom-up process, we can easily fill the 100 um-wide and 250 um-deep TSV without the void formation, ensuring the good electrical characteristics for the Cu TSV pairs.

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Figure 8. The resistance measurement results of single TSV.

Figure 9. The capacitance measurement of 100 parallel TSVs with diameter of 100 μm and 25 μm

Figure 10. The leakage current measurement of 100 parallel TSVs with diameter of 100 μm and 25 μm.

For 100 µm-wide and 250 µm-deep Cu TSV, the electrical measurement results show the resistance of single TSV is 0.22 mΩ and the capacitance of 100 parallel TSVs is 591 fF, the same order of magnitude compared with the simulation results. The leakage current is smaller than 0.25 nA at the maximum voltage of 20 V, indicating the good isolation of oxide layer. In addition, the measurement results with the diameter of 25 and 100 µm of Cu TSV are also presented in Figures 8-10. According to the RC electrical characteristics and leakage current, the integration of Cu TSV by the bottom-up process can be achieved. After the fabrication of Cu TSV, the Cu/Sn metal line was deposited around the 1210 silicon

package for next step wafer bonding to protect quartz blank against mechanical attack and mass loading from moisture or other contamination.

PKG bonding, Quartz Blank, Hermetic and Reliability Investigation

The thermo-compression eutectic bonding process was applied to integrate Si-cap cavity unit and TSV substrate. The formation of intermetallic compound ensures that the quartz blank was well-protected. The bonding conditions for the Cu/Sn metal layers are shown in Figure 11.

Figure 11. The conditions of thermo-compression bonding for Cu/Sn metal layer.

Figure 12. (a) The cross-sectional view of Si-cap and TSV substrate connected by the Cu/Sn bonded structure; (b) Cu/Sn bonded layer; (c) the image of Si-cap and TSV substrate.

Some reliability concerns may happen during the thermo-compression bonding process. The mismatch of CTE (thermal expansion coefficient) between silicon and metal layer may cause residual stresses and eventually lead to the poor bonding quality during manufacturing and hermetic test. In addition, the bonding conditions, such as material properties, geometry, bonding temperature and bonding pressure will significantly affect the bonding process and bonding quality of the package. However, as shown in Figure 12, the Cu/Sn bonded layer presents good bonding quality without voids and cracks. The Cu3Sn phase was formed in the bonded layer after the thermo-compression bonding process (Figure. 13). The proposed material provides good reliability and excellent bonding quality for crystal package.

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Figure 13. The Cu3Sn phase was formed in the bonded layer after the thermo-compression bonding process.

Ag conductive adhesion was deposited on Cu TSV of Si substrate to attach crystal quartz, which was the so-called quartz blank mount process. After sputtering the electrode on the quartz blank, the quartz blank attached on the Si substrate was cured in a high temperature vacuum oven. Figure 14 shows the images of quartz blank mount process and the adhesion between crystal quartz and Ag conductive layer on Si substrate during thermal flow.

Figure 14. (a) The quartz blank mount process, (b) the SEM images for the adhesion between crystal quartz and Ag conductive layer on Si substrate.

The hermetic and reliability tests with thermal cycles are important tests for the crystal resonator. After wafer-level integration was completed (Figure 15(a)), the wafer was diced (Figure 15(b)) and the quartz crystal device was well-protected in the cavity of Cu/Sn bonded Si-cap and TSV substrate wafer. Then the test was begun by placing the samples in a chamber filled with helium gas at a pressure of 0.5 MPa for an exposure time of over 2 hrs (“helium bombing”). Then, the samples were unloaded from the bombing chamber followed by helium leak test using a mass spectrometer. This novel package successfully passed the hermetic leakage test (8.5*10-9 ~1*10-8 Pa m3/sec by He leak detector) [11], as shown in Figures 15(c) and (d). With passing the hermetic and reliability tests, this quartz crystal

resonator using 3D integration shows the robustness without leakage path, indicating its potential for future applications.

Figure 15. (a) The cross-sectional image of bonded Si-cap and TSV substrate; (b) OM profile of the final structure; (c) hermetic and reliability tests with thermal flow; (d) results of hermetic and reliability tests.

The Electrical Characteristics of 1210 Crystal resonator with Silicon Package

Figures 16 (a) and (b) show the IEC441-1 standard for measurement of quartz crystal resonator (IEC: International Electro-technical Commission), and the equivalent circuit for quartz crystal resonators with silicon package, respectively. Since this approach is used for conventional crystal package, such as SMD and metal cap type, it is applied for the measurement of this novel TSV-based resonator package.

Figure 16. (a) The standard for crystal measurement and (b) the equivalent circuit for quartz crystal resonators.

The measurement results of the capacitance for crystal resonator package (C0) by IEC444-1 method are shown in Figure 17. The average capacitance was less than 2.5 pF, showing the good electrical performance of this package.

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Overall, this crystal resonator package using 3D integration has been developed and investigated its electrical, hermetic characteristics, and reliability performance. The excellent results of this design, fabrication, and scheme show the potential for future crystal resonator products.

Figure 17. The measurement results of the capacitance for crystal resonator package. Conclusions

The optimum design, simulation, fabrication, and performance of a novel crystal resonator using 3D integration and Si packaging technologies are presented in this paper. Different from conventional crystal package such as SMD, metal cap type, and ceramic enclosures, this new approach for crystal package was designed with hermetically eutectic metal bonding, wafer thinning, and 3D TSV interconnection. The design of 1210 crystal quartz device with 3D integration is able to meet the requirements of the small form factor and cost effective technologies for advanced crystal resonator applications, and provides the solution for future crystal resonator products.

Acknowledgments The research was supported by TXC and National Science

Council through Grant No. NSC 101-2628-E-009-005. The authors acknowledge facility assistance from National Chiao Tung University and TXC.

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