SET's latest developments in Removal of Oxide

10/2010 - Oxide reduction becomes a critical function that is addressed by SET new patented confinement chamber usable in C2C and C2W applications.



 Semi-Open Confinement Chamber: Principle
 Fluxless Bonding Solution using Formic Acid Vapor
 Installed Customer Base
 Formic Acid Vapor: Efficiency Testing


Oxidation of metal surfaces is a persistent problem in device bonding. Because oxides generally adhere poorly to other metals or oxides, the bonding force must penetrate the oxide to achieve metal-to-metal cohesion. Not only does this increase the required bonding force, but the oxides may also raise the electrical resistance of the joint. Even after the device has been bonded, existing oxides may provide a convenient site for further oxidation, leading to reliability and performance problems.

For this reason, high quality and reliable bonding often requires an oxygen free environment to prevent oxide formation during the bonding sequence at elevated temperature. Some materials such as Indium form native oxides at room temperature which must be removed before bonding. It is therefore necessary to remove and prevent oxides in situ to achieve proper joining.

SET is the leader for bonding equipment for hybridizing infrared Focal Plane Arrays. In this application, the detector chip made of compound semiconductor material (i.e.: GaAs, HgCdTe, InP, etc) is attached to the CMOS readout circuit through very high density Indium Micro Bump Arrays.

Breaking through the oxide can be performed by mechanical scrubbing, but this method is only effective with large bumps at loose alignment tolerances. An alternative solution is to use liquid flux to reduce the oxide, but this requires an additional cleaning step to remove flux residues which becomes very difficult at small gaps between dice.

SET has developed a substrate chuck and a bond head with a localized confinement chamber which operates safely with reducing gases such as forming gas or formic acid vapor. This patented configuration has been successfully implemented on SET bonder models FC150 and FC300.

To preserve the standard capabilities of our hybridization equipment and especially the low contact force measurement applied to the components, SET has developed a “semi-open” confinement chamber with no hardware sealing. A non-contact virtual seal is used to ensure gas collection and prevent oxygen intrusion.

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 Semi-Open Confinement Chamber: Principle

The principle of the virtually sealed confinement chamber is described below. The non-contact virtual seal of the micro-chamber enables gas confinement for chip-to-chip or chip-to-wafer bonding under controlled atmosphere while preserving the alignment of the device with respect to its substrate.





 The process gas is injected through horizontal nozzles towards the device being bonded;
 an exhaust ring removes the process gas from the micro-chamber and into the gas exhaust line, keeping the gas out of the machine and the clean room;
 a nitrogen curtain is formed around the exhaust, ensuring that ambient air is not entrained into the micro-chamber by the Venturi effect;
 a cover attached to the bond head creates the confined micro-chamber.

This configuration enables three operation modes:

1. with an inert gas such as nitrogen to prevent oxide formation on bonding surfaces 
during the bonding sequence;

2. with forming gas to remove and prevent oxide formation:
 Well-suited for applications using AuSn such as optoelectronics assembly
(Laser bar, VCSELs, etc.);

3. with a highly efficient reducing gas such as formic acid vapor to remove oxides
prior to bonding
, thus ensuring good wetting and high quality solder joints:
Well-suited for application using Indium.


Main Features

Formic acid vapor for oxide reduction

Inert, process or forming gas

Virtually sealed chamber, with
no contact to bottom chuck

Chuck size: 50 or 22 mm



This confinement chamber can be used with pure nitrogen, nitrogen saturated with formic acid vapor or other process gases. The gas saturation is adjustable to meet the process requirements.


Key Benefits

High quality and reliable bonding

Reduced bonding forces and temperatures due to oxide-free surfaces

In-situ flux-less bonding eliminates the need for other cleaning steps

Higher yield and reliability by thorough removal of oxides

Fluxless Bonding Solution using Formic Acid Vapor

Formic acid vapor is injected during in-situ bonding, after alignment of the components (chips and substrates) and just before bonding. When the components are nearly in contact, the Formic Acid vapor is injected into this micro-chamber, removing any oxides and enabling low-force bonding. Both components remain secured during onto their respective supports during the entire process. The post-bonding accuracy varies from ± 0.5 to 3 µm according to machine configuration.

This technique has been developed to avoid using liquid fluxes especially for infrared focal plane array applications where these fluxes are extremely difficult to clean after bonding. Indium oxides are easily removed in the formic acid vapors. Other applications for this technique include:

optoelectronics i.e. laser bar application with AuSn,
3D IC’s with Copper-to-Copper processes
other applications where oxide removal is necessary before reflow.

 Installed Customer Base

SET introduced the first generation of this chamber in 2006, with subsequent generations focused on improving the concentration and efficiency of the system. Nearly 20 machines have been installed around the world in leading research institutes (i.e.: METUMEMS...) and industrial sites. It is used for military, space or commercial applications.

 Formic Acid Vapor Efficiency Testing






Formic Acid Vapor efficiency is demonstrated using wafer coupons with copper metallization.

1. First heating cycle with formic acid vapor (FAV) environment:
The Formic acid vapor prevents oxide formation while the sample is heated 
 30 seconds at 350°C – FAV at 2 bar, ~ 8 liters / minute

2. Second heating cycle in ambient air: 
Oxygen around the component contributes to heavy oxide formation
 3 seconds at 350°C – No gas injection / Ambient air environment

3. Third heating cycle with formic acid vapor environment:
The formic acid vapor is reintroduced in the chamber where the copper 
plated wafer coupon was initially oxidized.
The reducing efficiency of the
formic acid vapor is visible by naked eye only after few seconds.

 30 seconds at 350°C – FAV at 2 bar, ~ 8 liters / minute

The experiment was carried out using the SET FC150 Bonder. Watch the video sequence below to notice the benefits of the formic acid vapor system. With no formic acid environment, the copper plated wafer coupon is exposed to ambient air (oxygen) and formation of an oxide layer is shown by the copper turning black. The oxide removal of the formic acid vapor is clearly demonstrated by the color change of the copper plated sample. Just a few seconds after the formic acid vapor is introduced in the chamber, the copper layer becomes shiny again.


Video Sequence

 Bonding Process

 Chuck temperature was 300°C

 Formic acid vapor was mixed
with nitrogen using an
evaporator system

 Nitrogen pressure was 2 bar
and the flow rate was adjusted 
to ~8 liters/min



At SET, we believe that reducing gases such as formic acid vapor are the best solution for removing oxides before bonding. Not only do these gases safely and efficiently prepare the surface for effective bonding at low forces, they do so without using liquid fluxes. This concept has been implemented on SET bonders for a variety of applications from research to industrial production, without sacrificing alignment accuracy or other modes of the tool’s operation.