Soon Wee Ho

High RF performance TSV silicon carrier for high frequency application

Three dimensional system-in-package (3D SiP) based on silicon carriers or interposer is a fast emerging technology that offers system design flexibility and integration of heterogeneous technologies. One of the key technologies enabler for silicon carrier is through silicon via (TSV). The development of 3D SiP will require the devices with different functionality operating at high frequency to be densely packed on the silicon substrate. However, silicon substrate is usually of low resistivity, when a high frequency signal is transmitted vertically through the substrate via, significant signal attenuation can occur that leads to poor RF performance. In this paper, a coaxial TSV structure in silicon carrier is presented for high frequency applications. The coaxial TSV is able to suppress undesirable substrate loss as well as provide good impedance matching. Electrical modeling of coaxial TSV structure was carried out to obtain the required geometries for impedance matching. Three different types of test vehicles were fabricated; Cu-plug TSV in both low (~10 Omega-cm) and high resistivity (~4000 Omega-cm) silicon substrate, and coaxial TSV in low resistivity silicon substrate. The S-parameters of the via structure of the test vehicles were measured from 100 MHz to 10 GHz. The measured results show that the coaxial TSV structure is able to suppress silicon substrate loss and provide good RF performance compared to Cu-plug TSV structure.

Study of 15µm pitch solder microbumps for 3D IC integration

Developments of ultra fine pitch and high density solder microbumps and assembly process for low cost 3D stacking technologies are discussed in this paper. The solder microbumps developed in this work consist of Cu and Sn, which are electroplated in sequential with total thickness of 10µm; The under bump metallurgy (UBM) pads used here is electroless plated nickel and immersion gold (ENIG) with thickness of 2µm. Accordingly, joining of the two Si chips can be conducted by joining CuSn solder microbumps to ENIG UBM pads or CuSn solder microbumps to CuSn solder microbumps. The first joining can only be done with chip to chip assembly whereas the second joining has the potential for chip to wafer assembly. Assembly of the Si chips is conducted with the FC150 flip chip bonder at different temperatures, times, and pressures and the optimized bonding conditions are obtained. After assembly, underfill process is carried out to fill the gap and a void free underfilling is achieved using an underfill material with fine filler size.

Fabrication of High Aspect Ratio TSV and Assembly With Fine-Pitch Low-Cost Solder Microbump for Si Interposer Technology With High-Density Interconnects

Fabrication of high aspect ratio through silicon vias (TSVs) in a Si interposer and fine pitch solder microbumps on a top Si die is discussed in this paper. Chip stacking result of the Si interposer and the top Si die is also presented. TSVs with 25 μm in pitch and aspect ratio higher than 10 are etched with BOSCH process. To avoid difficulties in wetting the sidewall of the TSVs, bottom-up plating method is used to fill the TSVs with Cu. In order to fill the TSVs from bottom, the TSVs are first sealed from the bottom by plated Cu with plating current of 1 A. The plated Cu is used as a seed layer and bottom-up plating is then conducted with plating current of 0.1 A. Good filling without voids or with only tiny voids has been achieved. Electroless nickel/immersion gold is plated on top of the TSVs as under bump metallurgy pads. On the top Si die, Cu pillars/Sn caps with 16 μm in diameter and 25 μm in pitch are fabricated with electroplating method. After chip stacking, interconnections are formed between them through the solder microbumps and the TSVs.

Development of Fine Pitch Solder Microbumps for 3D Chip Stacking

Developments of ultra fine pitch and high density solder microbumps for advanced 3D stacking technologies are discussed in this paper. CuSn solder microbumps with 25 ¿m in pitch are fabricated at wafer level by electroplating method and the total thicknesses of the platted Cu and Sn are 10 ¿m. After plating, the micro bumps on the Si chip are reflowed at 265°C and the variation of bump height measured within a die is less than 5%. The under bump metallurgy (UBM) layer on the Si carrier used is electroless plated nickel and immersion gold (ENIG) with total thickness less than 5 ¿m. Assembly of the Si chip and the Si carrier is conducted with the FC150 flip chip bonder at different temperatures, times, and pressures and the optimized bonding conditions are obtained. After assembly, underfill process is carried out to fill the gap and a void free underfilling is achieved using an underfill material with fine filler size.

3D packaging with through ilicon via (TSV) for electrical and fluidic interconnections

In this paper a liquid cooling solution has been reported for 3-D package in PoP format. The high heat dissipating chip is mounted on a silicon carrier, which has copper through-silicon via for electrical interconnection and through-silicon hollow via for fluidic circulation. Heat enhancement structures have been embedded in the chip carrier. Cooling liquid, de-ionized water is circulated through the chip carrier and heat from the chip is extracted. The fluidic channels are isolated from electrical traces using hermetic sealing. The research work has demonstrated 100 W of heat dissipation from one stack and total of 200 W from two stacks of the package. The fluidic interconnections and sealing techniques have been discussed.

Solutions Strategies for Die Shift Problem in Wafer Level Compression Molding

Die shift problem that arises during the wafer molding process in embedded micro wafer level package fabrication was systematically analyzed and solution strategies were developed. A methodology to measure die shift was developed and applied to create maps of die shift on an 8 inch wafer. A total of 256 dies were embedded in an 8 inch mold compound wafer using compression molding. Thermal and cure shrinkages of mold compound are determined to be the primary reasons for die shift in wafer molding. Die shift value increases as the distance from the center of the wafer increases. Pre-compensation of die shift during pick and place is demonstrated as an effective method to control die shift. Applying pre-compensation method 99% of dies can be achieved to have die shift values of less than 40 μm. Usage of carrier wafer during wafer molding reduces the maximum die shift in a wafer from 633 μm to 79 μm. Die area/package area ratio has a strong influence on the die shift values. Die area/package area ratios of 0.81, 0.49, and 0.25 lead to maximum die shift values of 26, 76, and 97 μ.m, respectively. Wafer molding using low coefficient of thermal expansion (7 × 10-6/°C) and low cure shrinkage (0.094%) mold compounds is demonstrated to yield maximum die shift value of 28 μm over the whole 8 inch wafer area.

Development of coaxial shield via in silicon carrier for high frequency application

System-in-package (SiP) based on silicon carriers is a fast emerging technology that offers system design flexibility and integration of heterogeneous technologies. One of the key technologies enabler for silicon carrier is through wafer interconnects. The development of SiP will require the devices with different functionality operating at high frequency to be densely packed on the silicon substrate. However, silicon substrate is usually of low resistivity, when a high frequency signal is transmitted vertically through the substrate via, significant signal attenuation can occur that leads to substrate crosstalk and poor RF performance. In this paper, a novel coaxial shielded via in silicon carrier is presented for high frequency applications. Electrical modeling was carried out to obtain the required geometries for optimum performance. The coaxial shield via is able to suppress undesirable substrate crosstalk between vertical interconnects as well as provide excellent RF performance. The detailed fabrication process is also presented. A negative tone SU-8 photoresist is used as the dielectric for the coaxial shield via structure. A test vehicle is fabricated on 8-inch, 10 Omegamiddotcm resistivity silicon wafer with a target of achieving a transmission coefficient, S21 of greater than -0.5 dB at 40 GHz. SU-8 dielectric of approximately 112 mum thickness was deposited on the via sidewall of a 300 mum diameter through wafer via holes, and the via-holes filled with copper using bottom up electroplating approach to achieve a radius ratio, n of 4

3-D Packaging With Through-Silicon Via (TSV) for Electrical and Fluidic Interconnections

In this paper, a liquid cooling solution has been reported for 3-D package in package-on-package format. A high heat dissipating chip is mounted on a silicon carrier, which has copper through-silicon via (TSV) for electrical interconnection and hollow TSV for fluidic circulation. Heat transfer enhancement structures have been embedded in the chip carrier. Cooling liquid, de-ionized water is circulated through the chip carrier and heat from the chip is extracted. The fluidic channels are isolated from electrical traces using hermetic sealing. The research work has demonstrated liquid cooling solution for 100 W from one stack and total of 200 W from two stacks of the package. The fluidic interconnections and sealing techniques have been discussed.

Embedded Wafer Level Packaging for 77-GHz Automotive Radar Front-End With Through Silicon Via and its 3-D Integration

In this paper, a 77-GHz automotive radar sensor transceiver front-end module is packaged with a novel embedded wafer level packaging (EMWLP) technology. The bare transceiver die and the pre-fabricated through silicon via (TSV) chip are reconfigured to form a molded wafer through a compression molding process. The TSVs built on a high resistivity wafer serve as vertical interconnects, carrying radio-frequency (RF) signals up to 77 GHz. The RF path transitions are carefully designed to minimize the insertion loss in the frequency band of concern. The proposed EMWLP module also provides a platform to design integrated passive components. A substrate-integrated waveguide resonator is implemented with TSVs as the via fences, and it is later used to design a second-order 77-GHz high performance bandpass filter. Both the resonator and the bandpass filter are fabricated and measured, and the measurement results match with the simulation results very well.

3D interconnection process development and integration with low stress TSV

The 3 D interconnect technology with Thru Silicon Via (TSV) have gained tremendous advancement in recent years. Final adoption of TSV technologies requires a robust and cost competitive TSV processes. Sidewall plated TSV with polymer filling can reduce half of total process steps from TSV copper (Cu) seed deposition to front-via1 expose. TSV plating time can be reduced ~ 60% for sidewall plated TSV with polymer filling. Costly Cu removal process through chemical mechanical polishing (CMP) can be skipped in sidewall plated TSV with polymer filling process. Wafer warpage and bow for sidewall plated TSV with polymer filling were shown to be ~70% and ~94%, respectively lower than solid Cu filled TSV. Thermal-mechanical simulation show 20% and 42% reduction of shear and bending stress respectively in the case of sidewall plated TSV with polymer filling.