Pumping systems
Semiconductor processes often require low pressure environments around the wafer surface,
i.e., that we evacuate nearly all the gases inside the reaction chamber using vacuum
pumping systems to pressures typically a million times lower than atmospheric pressure: 1
atmosphere pressure is 760 torr, whereas many semiconductor processes operate at only
0.1-10 torr.
In addition, for low pressure or even atmospheric pressure processes, the wafer environment must contain only gas species needed for the process. Any other reactive species (like spurious water vapor, organics, or oxygen) will degrade process and material quality. Thus it is essential to evacuate all atmospheric gas components to very low levels first, before intentionally introducing the gas species needed for the process.
A variety of pump types and configurations are used in semiconductor manufacturing, including: rotary vane mechanical (mech) pumps which operate from atmospheric pressure to low pressure (10-2 to 10-3 torr); Roots blower and molecular drag pumps which enhance flow rates and gas handling capability of mechanical pumps; and other pumps (turbomolecular, cryogenic, and diffusion pumps) to achieve very low pressures (10-3 to <10-9 torr) when used in conjunction with mech pumps.
Gas inlet systems
While pumping systems enable the evacuation of gases from reaction chambers and other
sealed vessels, it is also necessary to have means to introduce gases into sealed chambers
for two reasons. First, specific gases at particular pressures are required in order
to carry out processes, both chemical and physical in nature. Thus means must be supplied
for introducing controlled flows of gas from specific sources, including reactive species
such as oxygen, fluorocarbons, hydrogen, or silane, and also inert species like argon or
nitrogen. Second, and much more mundane, once a chamber has been evacuated to even mildly
subatmospheric pressures, the atmospheric pressure outside the chamber is no longer
balanced by the pressure inside, and it will be very difficult to remove flanges (covers)
from the chamber in order to insert or remove a wafer.
Gas inlet may be accomplished by a simple venting system, in which a cutoff valve is opened to admit gas up to a required pressure. This approach is regularly used as a means to bring the chamber pressure up to atmospheric pressure before opening the chamber.
Much more sophisticated flow control systems are commonly used in semiconductor manufacturing processes. These incorporate sensors which measure the amount of gas flow (calibrated for the gas species), as in a mass flow sensor. Frequently, the mass flow sensor is combined with actuators to change the flow amount (e..g, through a variable-conductant valve) and electronic control systems for dynamic regulation of flow or pressure, forming a mass flow control (MFC) system. Many such MFC's are regularly employed in semiconductor process equipment.
Pressure
gauges
Pressure measurement methods are essential in semiconductor process equipment, because
process results depend on the total operating pressure and the individual contributions to
it from various gaseous species.
Total pressure indicates the sum of contributions from all gases which may be present in a given chamber. Different approaches to measuring pressure are valid only in certain pressure regimes, typically not broad enough to encompass the large dynamic range of pressures encountered, from very low pressure (around 10-9 torr) to atmospheric pressure (760 torr). Therefore, combinations of total pressure gauges are employed, including thermocouple or Pirani gauges), capacitance manometer gauges, ionization gauges, and others.
Also crucial in semiconductor manufacturing processes are partial pressure measurements, i.e., the determination of the pressure associated with various gaseous species individually. Most commonly this is accomplished by residual gas analyzers (RGA's), which are essentially mass spectrometers which exploit ionization in order to determine the charge/mass ratio of the ions generated.