Nitrogen is most commonly used in the packaging process of optical devices. Another common gas is helium, which is used to work with nitrogen.
The long-term working environment of lasers and detectors needs to be isolated from oxygen and water. The majority of the lasers and detectors used in our optical modules are made of composite materials.Very few of them are made of elemental semiconductors. Compounds are materials formed by chemical reactions of several different materials, such as InP, InGaAs, InGaAsP, GaAs, etc. When they encounter oxygen or water molecules, they will produce new chemical reactions, so oxygen and water need to be isolated.
There are two ways of implementing isolation methods:
One of them is that the chip is placed in a "sealed independent" space, such as a TO can or a Golden Box package with a sapphire window. That is often referred to as hermetic packaging.
Another one is to coat the surface of the chip with a water- and gas-blocking film, such as a silicon oxide film, a silicon nitride film, an aluminum oxide film, and so on. That is what we often call the "passivation film" in non-hermetic packaging. Silicon oxide is a common main component of glass, silicon nitride is a common ceramic material, and aluminum oxide is also a common ceramic material, just like the process of our enamel tank is to hang a layer of ceramic slurry outside the iron tank to prevent iron from rusting (rust is the oxidation reaction of iron).
The TO can or golden BOX package also has two options. The first option can use a "vacuum" cavity to pump out all the gases inside, including oxygen and water. This method has a potential for failure. If the air pressure in the cavity is low and the external atmospheric pressure generates pressure outside the airtight cavity, this is likely to cause air leakage.
The second option is to fill the cavity with an "inert gas", which will not react with our materials and can achieve internal and external pressure balance.
How do I choose inert gas? For industrial manufacturing, the cheaper the better.
Nitrogen is the cheapest inert gas; 78% of the air is nitrogen, and 21% is oxygen. The oxygen in the air can be removed at a very low cost, leaving only nitrogen. Nitrogen gas is filled in the airtight cavity, that is, the golden box or TO Can.
Nitrogen is blown outside when bonding aluminum pads of chips such as gold wire and TIA/DRV/silicon photonics to prevent contact between aluminum and oxygen.
When doing the power test of the 5G fronthaul optical module and laser of CWDM6, it is necessary to blow nitrogen into the integrating sphere because the two wavelengths of CWDM6 are just in the absorption band of water molecules, and nitrogen is used to drive away the water vapor in the measurement environment.
Nitrogen gas should be blown during laser eutectic welding to prevent the tin in the gold-tin eutectic material from being oxidized.
When the chip is stored, it should be placed in a nitrogen cabinet to prevent the die from contacting with oxygen and water vapor, and to prevent the plastic in the plastic-packaged chip from absorbing water vapor, and so on.
Although this low-cost nitrogen production cannot obtain very pure nitrogen, it can drive off oxygen to an extremely low content and obtain process improvements. In a few applications, if the extremely low oxygen content cannot meet the needs of product manufacturing, other additional processes can be used to fix it.
Now that there is a low-cost inert gas such as nitrogen, why do we need to equip another inert gas, "helium"?
Pressing helium, usually in the process of air tightness testing, presses helium into the cavity.
After the optical device is hermetically sealed, it is necessary to check whether there is a sealing effect, and another inert gas can be selected to cooperate.
Helium is the best partner. Firstly, they are all inert, and they will not affect the function of our optical chip. Secondly, nitrogen is cheap, and it is given priority to be used in large-volume routine operations, while helium is expensive and only used for testing.
Thirdly, nitrogen and helium have a "mass" difference. easy to separate. Nitrogen is heavy, and helium is light. When the two are ionized under the action of an electric field, there are obvious differences in the landing positions of helium and nitrogen. This is the basic working principle of a mass spectrometer.
If the airtight optical device is well sealed, the nitrogen gas in the optical device will not come out, and the helium gas that is pressed in from the outside will not enter the optical device. The two do not overlap. But if the airtightness is not good, the optical device that was originally filled with nitrogen will now have helium.
Take out the optical device separately, and after opening the cavity, test the two gas contents inside (some can also test the outside gas content) to see if there is any aliasing between the two.
In theory, the gas in the cavity or the gas outside the cavity should be single, but if there are two types, it means that there is a gas leak.
The gas is ionized first, and the ionized gas moves under the action of the electric field. This is the ion wind, heavy nitrogen, light helium, separation, and then quantitative analysis to complete the air tightness test.
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