100 years of enabling

November saw the 100th anniversary of the vacuum tube that led to the birth of the electronics industry. David Ridsdale looks at how the vacuum continues to be a vital key for manufacturing both now and in the future.

The ability to create a vacuum has been a cornerstone of the electronics industry. The first technical use of a vacuum was in the 1600s when the vacuum pump was invented by Otto von Guericke in Germany. The next major step took over 200 years when Edison invented the light bulb. His work then inspired John Ambrose Fleming in 1904 in his creation of the vacuum tube, the birth of the electronics industry. Over time the vacuum tube was replaced in electronic circuitry but the vacuum was still essential in manufacturing with vacuum pumps providing all the gases and fluid materials to the industrious tools.

At the beginning of the 21st century it is only apt that vacuum should once again be a key factor in the future of the industry. The industry thrives on the scalable shrinking of designs and devices to produce ever more powerful microelectronic devices.

As the IC devices shrink the tools and manufacturing environment becomes more difficult as the most minute particles will destroy any manufacturing aim. Almost every part of high end microelectronic manufacturing must occur in a vacuum. This has meant that every tool chamber must provide an environment as void of as many particles as technically possible. With the advent of the super turbo vacuum pumps, the pumps have become smaller and quieter enabling them to be close to the tool and free up vital and valuable floor and underfloor space in the fab.

This has also meant a huge increase in the number of pumps on a fab floor. All companies look at ways to reduce costs, ensure constant vacuum pressure and reduce any floor space. Vacuum pumps continue to drive the flow of gases and fluid materials as well as provide the manufacturing environment required.

Initially, attempts by semiconductor manufacturing companies to get wafer fabrication processes under control were aimed at moving the process into an environmentally-controlled "clean room". By filtering the air, and carefully controlling the materials allowed into the clean room, the amount of particulation was reduced. By 1970, a Class 1000 clean room was considered quite good.

However, some of the equipment - and operators moving around in "bunny suits" - still created a large number of particles. To avoid this problem, it was necessary to build even cleaner areas inside the clean room. This was done with laminar flow hoods and other devices, in an attempt to reduce the number of particles that can come into contact with the silicon wafers. In 1973-1975, Class 100 areas within the clean room were considered good.

In the 1980s, equipment manufacturers began moving their semiconductor manufacturing processes into vacuum chambers. While the vacuum environment may be a part of specific processes, it also has the advantage of not supporting particle motion.

It is likely that the future trend will be to make large sections of the process line, or even the entire process line, operate in a vacuum. The system will essentially be a long pipeline with many segments that can be sealed off from one another. Once the wafers have moved into a segment or section, that section is sealed, the appropriate process is accomplished and the wafers are then transported to the next section.

By having large numbers of processes linked into a series of vacuum chambers, several advantages are gained. The lack of air in the system prevents transport of contaminating particles, and the time spent pumping down the process chambers to the required levels of vacuum can be eliminated. The process becomes more of a continuous flow, as opposed to a batch mode.



Field emitter vacuum tubes

In the early years of the 21st century there has been renewed interest in vacuum tubes, this time in the form of integrated circuits. The most common design uses a cold cathode field emitter, with electrons emitted from a number of sharp nano-scale tips formed on the surface of a metal cathode.

Their advantages include greatly enhanced robustness combined with the ability to provide high power outputs at low power consumptions. Operating on the same principles as traditional tubes, prototype device cathodes have been constructed with emitter tips formed using nanotubes and by etching electrodes as hinged flaps that are stood upright by a magnetic field.

Such integrated microtubes may find application in microwave devices including mobile phones, for Bluetooth and Wi-Fi transmission, in radar and for satellite communication. Presently they are being studied for possible application in flat-panel display construction.