Load-lock Vacuum Systems

A load-lock is a vacuum chamber used in the vacuum and semiconductor industries for loading devices like semiconductor wafers and transferring them from ambient air pressure to the main vacuum processing chamber. In semiconductor fabrication, the main process chamber is kept at high vacuum pressure and is not vented to ambient pressure during processing. To protect this environment, an auxiliary vacuum chamber is used to isolate the wafers from the main process chamber during ventilation. These chambers are essential for semiconductor fabrication, providing a clean, stable environment for handling sensitive materials.

The load-lock chamber cycles between atmospheric pressure and the required vacuum level to transfer wafers to the processing vacuum chamber via a buffer transfer chamber. Precise control of vacuum pressure relative to ambient pressure in the load-lock chamber is crucial to prevent ambient air from rushing in, which could introduce particulate contamination. Such contamination increases pump-down times and the risk of particles affecting both the wafers and the vacuum system itself.

Load-lock pumping cycle

To control pressure during the evacuation cycle, a Pirani heat-loss gauge is typically utilized to measure the vacuum gas pressure and to provide a relay set-point control signal once the vacuum pressure between the load-lock and transfer chamber is equalized. The wafer is mechanically transferred to the transfer chamber and a processed wafer is transferred back to the load-lock. The load-lock is typically ventilated with Nitrogen gas to ambient pressure or a pressure value above ambient pressure to prevent air contaminating the load-lock chamber.

Venting to atmosphere 

To control the pressure in the venting cycle a vacuum sensor measuring relative to ambient pressure, commonly known as a gauge sensor, is used. The gauge pressure measurement can be achieved by having a differential pressure sensor where one side of the diaphragm is exposed to atmosphere and the other side is exposed to ambient pressure. Another approach is to use an absolute barometric sensor and an absolute diaphragm sensor in vacuum and subtract the two measurements to achieve a gauge pressure value.  The advantage of using a gauge sensor to control venting is that the load-lock pressure can be accurately equalized to zero differential pressure between the load-lock vacuum chamber and ambient pressure independently of variation in ambient pressure due to weather changes.

Load-lock applications

Fast cycle time of the load-lock is important to optimize the wafer processing cost. Load-locks play a vital role in enhancing the precision, efficiency, and reliability of semiconductor fabrication processes, ultimately leading to the production of high-quality semiconductor devices. Use of accurate and modern load-lock pressure control can shorten cycle times. 

Load-lock vacuum systems are not only used in the semiconductor industry, but also in analytical equipment, like scanning electron microscopes, where samples are transferred from ambient pressure to the analysis vacuum chamber.

In a PVD (Physical Vapor Deposition) system, it can also be an advantage to maintain the processing deposition chamber under permanent vacuum pressure and only having to vent an auxillary load-lock vacuum chamber.

Single-chamber PVD systems have a large door system to the ambient pressure to load the carousels with the coating objects. The large door surface requires accurate control of the venting process to both prevent over pressurization of the vacuum chamber and to equalize the pressure to open the door.

Many vacuum processes and types of vacuum systems, where samples, wafers, test objects, substrates or other devices need to be transferred from ambient air to the vacuum system, can benefit from having an auxiliary load-lock system to ensure fast cycle-times, system uptime, reduced particulate contamination and water vapor contamination.

Sens4 has invented a new multi-sensor wide-range combination transducer for control of load-lock pressure cycles, which enables accurate ventilation to ambient pressure and accurate pump-down control to transfer vacuum pressure level. It also offers a record-breaking wide range that, in some applications, can eliminate the need for hot or cold cathode combination gauges for load-lock applications, where low transfer pressure level is required.

The SmartPirani™ ATM transducer can replace an atmospheric switch, a diaphragm vacuum transducer and a heat-loss Pirani transducer used on older load-lock systems – or it can be a performance upgrade on load-lock vacuum systems using legacy load-lock transducers.