Why are some semiconductor device production processes carried out in a vacuum?

  In the production of semiconductor devices, in addition to the need for an ultra-clean environment, some processes must also be carried out in a vacuum. The atmospheric environment in which we live is filled with a large number of nitrogen, oxygen, and other various gas molecules, and these gas molecules are in motion all the time. When these gas molecules move to the surface of the object, some of them will stick to the surface of the object. This doesn't make much difference in everyday life.

However, in the production process of semiconductor devices that have extremely high demands on the surrounding environment, these subtle changes can cause various problems in production. Every semiconductor device contains many layers of various materials, and if gas molecules are mixed between these different layers of materials, the electrical or optical properties of the device can be destroyed. For example, when it is desired to grow another layer of crystal on the crystal layer (called epitaxy), the gas molecules adsorbed on the surface of the underlying crystal will prevent the atoms above from being arranged in an orderly manner according to the lattice structure, resulting in the introduction of a large number of defects in the epitaxial layer. , in severe cases, even no crystal can be grown, but only polycrystalline or amorphous with the disordered atomic arrangement.

Under atmospheric pressure, every point on the crystal surface will be hit by hundreds of millions of gas molecules every second. Therefore, to obtain a clean crystal surface, the density of gas molecules is usually reduced to hundreds of millions of atmospheric densities. Only one part is required, that is, a vacuum environment needs to be obtained. To this end, people have created large and small airtight containers and invented various vacuum pumps to pump air out of these airtight containers, making their interiors a vacuum environment.

Many semiconductor devices, such as optical disc players (CD, VCD, and DVD) and semiconductor lasers used in optical fiber communications, microwave integrated circuits in radar or satellite communication equipment, and even many ordinary microelectronic integrated circuits, have a considerable part of the production process. is carried out in a vacuum container. The higher the vacuum level, the better the performance of the fabricated semiconductor device. Today, many high-performance semiconductor devices are fabricated in ultra-high vacuum environments. To obtain the so-called ultra-high vacuum, the density of gas molecules in it is only one-hundred-billionth to one-hundred-trillionth of that in the atmosphere!

To obtain an ultra-high vacuum environment, a very complex and expensive pumping system is required. In addition, in the processing of semiconductor devices, the material needs to be irradiated and bombarded with particles such as electron beams, ion beams, and molecular beams. In the atmosphere, gas molecules collide with these particles, greatly shortening their travel distances, and as a result, the vast majority of particles do not reach the surface of the material.