Tutorial #113
George Riley

INTRODUCTION

As components shrink and component spacing decreases, the advantages of oxide-free bonding become more difficult to achieve. The recent introduction of a non-contact localized confinement chamber removes and controls oxidation without disturbing delicate component or their alignments.

Oxidized bonding surfaces adhere poorly, requiring higher bonding forces for good metal-to-metal contact. Oxides remaining in the completed joint may both increase the electrical contact resistance of the joint and seed future oxidation. Such residual oxidation threatens long-term reliability.

Copper, a favored material for 3-D assemblies, forms oxides immediately on exposure to air. Indium, often used in bonding large sensor arrays, has a native oxide that must be removed to achieve proper bonding.

Oxides may be removed with liquids or special fluxes, but these penetrate small spaces poorly. They also require a cleaning step after the bonding sequence.

Cleaners and cleaning products can be difficult to remove, leaving residual foreign material. Oxidation of the cleaned surface resumes immediately upon exposure to air.

Confinement Chamber

SET has introduced a local confinement chamber as part of the bonding tool. It creates a non-contact virtual seal to confine the process gas flowing through the chamber.

A nitrogen curtain prevents ambient air intrusion, while an exhaust ring removes the process gas from the chamber and the clean room.

Figure 1 is a cross-section view of the chamber in chip-to-chip bonding configuration. The process gas enters the chamber through horizontal nozzles at the bottom.

The process gas and nitrogen leave the chamber through the exhaust at the middle left. The nitrogen flowing in also flows out at the top, preventing ambient air from reaching the chamber, even though there is no mechanical seal.

Figure 1  Cross-section of the chamber in chip-to-chip configuration.

Operating Modes

The chamber may configured for chip-to-chip or chip-to-wafer assembly, with three operating modes:

• Inert Gas Mode: supplying nitrogen or other inert gas to prevent oxide formation during bonding.
• Gas Mode: supplying formic gas or other efficient reducing gas to prevent further oxidation.
• Reducing Gas Mode: supplying formic gas or other efficient reducing gas to remove oxidation prior to bonding, for good wetting and superior joints.

Effectiveness

Figure 2 shows the effectiveness of formic acid vapor (FAV) on removing oxide from a copper coupon. The coupon is sequentially exposed first to FAV, then to air, then FAV.

Figure 2. Demonstration of FAV effectiveness in removing oxide from copper.

(a) 30 seconds at 350 deg-c, FAV flow 8 liters per minute at 2 bar pressure.

(b) 3 seconds at 350 deg-c, no gas injection, ambient air. Observe the oxidized surface.

(c) FAV introduced at 350 deg-c, flow 8 liters per minute at 2 bar pressure.

Fluxless Bonding Process

The components are aligned in the bonder and brought nearly into contact. FAV is injected into the micro-chamber, removing any oxides, while the components remain securely on their respective supports.

Upon completion of oxide removal, the components are readily joined using low-force bonding.

Applications

Fluxless bonding removes Indium oxides, allowing low-force bonding of large focal-plane arrays. Other applications include with AuSn solder in optoelectronics applications such as laser bars. It is also excellent in 3D assemblies for copper-to-copper bonding.

Summary

SET’s versatile contactless confinement chamber quickly and efficiently removes oxides, permitting strong low-force bonding without liquid cleaning for a wide variety of high-precision applications.

FOR MORE INFORMATION

K. Cooper, M. Stead, G. Lecarpentier, & J-S Mottet, “Flip Chip Die Bonding: An Enabling Technology For 3D IC Integration,” Proceedings of the 7th Annual International Wafer-Level Packaging Conference, October 2010, pp 55 – 58.