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| Richard C. Kullberg
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Prior articles in this series discussed the sources of water vapor in hermetically sealed packages and the adverse consequences this water can cause for your devices. Having identified the issue, now we need to determine how to remove the water and solve the problems it causes.
The issue of water and its removal goes all the way back to the days of Thomas Edison late in the 19th century. In fact, Edison’s assistant Malignani coined the very language used for removing contaminants like water, e.g. ‘getting’ the contaminant with ‘getters’. In 1882. Malignani developed the technique of coating components of incandescent lamps with red phosphorous. Red phosphorous reacts with, or getters, water vapor, thereby breaking the water-tungsten cycle that limits lamp lifetime. This process is still used today in the lamp industry, over a century later.
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| Robert K. Lowry
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There are basically two types of getter materials of interest for controlling water vapor contamination in microelectronic packages. They break down by how they react with the water, or even more simply, whether they take the water on a one-way trip out of the system to be locked up by the getter, or allow a two-way trip back into the vapor phase.
Materials that take the water on a two-way trips include materials like zeolites. These can be viewed as highly absorbing sponges with lots of surface area for physisorption to occur on.
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| typical zeolite structure
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Why do we say that water can go on a two way trip into and out of these sort of materials? It is a simple issue of vapor pressure. Rather then get into an involved discussion and rewrite what is already in the literature, just know that under certain conditions of temperature, pressure, and the amount of water sorbed on a zeolite or similar material; the water can evaporate off again and return to cause you trouble.
No doubt, zeolites and other physisorbants are very useful materials in electronics packaging, and are used every day in industry. However, in many systems taking the water on a one-way trip and locking it up completely out of action is desirable.
The classic way to take water on a one-way trip out of a package is to use a metal getter like barium, titanium, or zirconium. We have all seen these types of getters in action as they are used in old style CRT TV sets and electronic tubes (the barium getter is the silver coating you see on the side or end of an electronic tube.
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| barium getter in an electronic tube
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Metal getters work by a process of first physisorption, just like zeolites do, but once the water molecule is settled in on the metal surface it is split into hydrogen and oxygen. The oxygen reacts with the metal surface, the hydrogen goes into solution in the underlying lattice structure of the metal itself, and the water is irreversibly removed
But if you just put a piece of metal getter in your system nothing will happen. This is the downside of metal getters. As delivered they are passivated, rendered chemically inactive. This is necessary in order to be able to manufacture and deliver them to point of use. To render them useable they must be ‘activated.’ Activation is achieved by heating the getters to very high temperatures that can range from 300ºC to 1000ºC! These temperatures either evaporate a useful film of the getter in the case of barium and titanium or cause the depassivation of the surface of zirconium and titanium nonevaporable getter (NEG) alloys. Careful consideration of the issues thermal activation brings to the table will quickly show why these materials are typically used only in vacuum or noble gas ambients. You don’t want to burn them up!
It is pretty self-evident that being able to take water out of a package on a one way trip without burning the plant down would be a very neat thing to do. Unfortunately industry isn’t there yet. ‘Yet’ is the key word, but that is a story for another day!
Robert K. Lowry (321-777-9949, www.electronic-materials.com) is a consultant/materials scientist with 39 years microelectronic industry experience and is also a principal with Arthur Jonath Associates (http://www.jonathassociates.com/).
Richard C. Kullberg (719-966-4296, rckullberg@vacuumenergyinc.com, brings 30 years of materials science and microelectronic industry experience to his product and business development work with Vacuum Energy, Inc. (http://www.h2getters.com/).
See previous articles written for this series in Semiconductor Packaging News
Where Does Water Come From? posted on June 15, 2009
STICTION posted on May 13, 2009
