Hiroaki Shindo

Nano-Function Paste for power semiconductors

The Nano-Function Paste (NFP) is a conductive paste contains Cu (or Cu composite) particles with coating and Low Melting Point Alloy (LMPA) particles. Interconnects using high Cu content (70 wt%) NFP had shown shear force after 300 deg. C storage (>1,000H) was better than that of nano-Ag paste. This paper is an analysis of low Cu content (20 wt%) NFP providing; graded Cu diffusion into LMPA, inhibition of IMC voids, well controlled self-alignment capability (surface tension) for power semiconductors and SMD interconnects.

Nano-Function materials for TSV technologies

This paper discloses that Nano-Function materials make TSV structure by printing technologies without CVD/PVD/Plating. For isolation layer forming, two types of TSV pattern had been examined. For metal fill, we adopted conductive paste or alloy plate contains nanomized alloys (Cu, Sn and additives) to fill via by less than 250°C condition. Re-melting temperature of the alloy is more than 300°C.

3DIC/TSV process developments by printing technologies

We have examined printing technology to be adaptable to TSV forming process such as isolation layer forming and via fill. The result shows good possibility. Isolation layer forming by silica paste fill had been confirmed to adaptable for CD 1.5μm to 15μm. Printing process in vacuum chamber by squeezing followed by centrifugal processing is a candidate even though it should be improved on fill-rate. We chose Nano-Function material for TSV fill which was initially developed for power semiconductor attachment. They filled into via also using vacuum circumstances. We confirmed good results to fill via array by the paste using glass substrate with 50μm pitch, 20-25μm diameter and 100μm depth.

New interconnection alloy metal for high bonding strength nano composite particles synthesized by nanomized method

In the metallization technology for power electronic device, it has been required to develop new material with low cost for operating high-temperature at 300 degree C and highly-reliable interconnection. We have developed new method to fabricate fine metal alloy particles with narrow distribution of particle size from 0.5 to 10μm. We called it as Nanomized method. The fine particles are composed of uniform structure dispersed metal alloy in nano-scale level and do not include void in the particle. The metal alloy particles fabricated by conventional atomized method include void inside particle and non-uniformly dispersed alloy components. We confirmed reliability and mechanical strength. We produced bonding material from the mixture of Cu particles and Sn-based alloy particles. The bonding strength of die chip on Al electrode reached 80 MPa as a top data, 50 MPa in average. The bonding strength of Sn-based alloy particle fabricated from conventional atomized method deteriorated less than 8 MPa after 500 hours later at 300 degree C.

Die Attach Material for Power Semiconductor Having Nano-Level Sn-Cu Diffusion Control

This paper discloses development and evaluation of die attach material using base metals (Cu and Sn) by three different type of composite. Mixing them into paste or sheet shape for die attach, we have confirmed that one of Sn-Cu components having IMC network near its surface has major role to provide robust interconnect especially for high temperature applications beyond 200°C after sintering.

New Cu paste with high bonding strength—Nano composite alloy particles synthesized by nanomized method

In the metallization technology for crystalline Si solar cell, it has been required to develop new material with low cost for high-temperature and highly-reliable interconnection. We have developed new method to fabricate fine metal alloy particles with narrow distribution of particle size from 0.5 to 10μm. We called it as Nanomized method. The fine particles are composed of uniform structure dispersed metal alloy in nano-scale level and do not include void in the particle. The metal alloy particles fabricated by conventional atomized method include void inside particle and non-uniformly dispersed alloy components. We confirmed reliability and mechanical strength. We produced bonding material from the mixture of Cu particles and Sn-based alloy particles. The bonding strength of die chip on Al electrode reached 80 MPa as a top data, 50 MPa in average. The bonding strength of Sn-based alloy particle fabricated from conventional atomized method deteriorated less than 8 MPa after 500 hours later at 300 degree C.

Cu-Sn based joint material having IMC forming control capabilities

This paper describes development of joint materials using only base metals (Cu and Sn) for power semiconductor assembly. The optimum composition at this moment is Cu8wt%Sn92wt% (8Cu92Sn hereafter) particles: pure Cu (100Cu hereafter) particles = 20:80 (wt% ratio), which indicates good stability under Thermal Cycling Test (TCT, −55°C∼+200°C, 20cycles). The composition indicated to be effective to eliminate voids and chip cracks. As an initial choice of joint material using TLPS (Transient Liquid Phase Sintering), we considered SAC305 might have good role as TLPS trigger. But, actual TCT results indicated that existence of Ag must have negative effect to eliminate voids from the joint region. Tentative behavior model using 8Cu92Sn and 100Cu joint material is proposed. Optimized composition indicated shear force 40MPa at 300°C. Re-melting point of the composition is 409°C after TLPS when there is additional Cu supply from substrate and terminal of mounted die.

Thermal resistance evaluation of die-attachment made of nano-composite Cu/Sn TLPS paste in SiC power module

In this paper, we demonstrate a high heat resistant bonding method by Cu/Sn transient liquid phase sintering (TLPS) method can be applied to die-attachment of silicon carbide (SiC)-MOSFET in high temperature operation power module. The die-attachment is made of nano-composite Cu/Sn TLPS paste. The die shear strength was 40 MPa for 3 × 3 mm2 SiC chip after 1,000 cycles of thermal cycle testing between −40 °C and 250 °C. This indicated a high reliability of Cu/Sn die-attachment. The thermal resistance of the Cu/Sn die-attachment was evaluated by transient thermal analysis using a sample in which the SiC-MOSFET (die size: 4.04 × 6.44 mm2) was bonded with Cu/Sn die-attachment. The thermal resistance of Cu/Sn die-attachment was 0.13 K/W, which was comparable to the one of Au/Ge die-attachment (0.12 K/W). The validity of nano-composite Cu/Sn TLPS paste as a die-attachment for high-temperature operation SiC power module is confirmed.

Fine pitch micro-bump forming by printing

We have examined printing technology which is adaptable to 3DIC bump-forming for (both front-side bump and back-side bump. The materials for bumping require several features for TSV process circumstances and 3DIC stacking followed by reflow. We chose Nano-Function material for the purpose which was initially developed for power semiconductor attachment. The result shows good possibility. 20μm bump pitch capability was confirmed.

New bonding material for power devices with high bonding strength for operating high temperature

In the metallization technology for power electronic device, it has been required to develop new material for operating high-temperature at 300 °C and interconnection with high bonding strength. We have developed new method to fabricate fine metal alloy particles with narrow distribution of particle size from 0.5 to 10μm. We called it as Nanomized method. The fine particles are composed of uniform structure dispersed metal alloy in nano-scale level and do not include void in the particle. We produced bonding material from the mixture of Cu particles and Sn-based nano composite solder particles. The bonding strength of die chip on Al electrode reached 80 MPa as a top data, 50 MPa in average after 500 thermal cycles from -40 to 250 °C.