Nitrogen gas has no toxic properties at atmospheric pressure, other than the possible hazard of suffocation due to the displacement of air. The only caution required is in handling liquid Nitrogen due to its extremely low temperature. Nitrogen gas makes up a major portion of the atmosphere, about 80% by volume with the other 20% being mostly Oxygen. Liquid Nitrogen is obtained by chilling air until it becomes a liquid and then removing the remaining gas which is mostly Oxygen. It is a colorless, odorless, flavorless, nontoxic, and almost totally inert gas and is colorless as a liquid. Nitrogen does not burn, and supports neither combustion or respiration.

Read all instructions carefully. Also, read and understand the instructions supplied by the Dewar manufacturer or any of the equipment manufacturers mentioned herein. Contact with liquid nitrogen or any cold gas may cause serious freezing (frostbite) injury. Do not touch liquid or cold metal surfaces with the bare skin. Although, the air around us is roughly 80% Nitrogen and 20% Oxygen, an excess of Nitrogen can replace all the air in a closed area and cause suffocation without warning. Use this apparatus only in a well ventilated area.

The method of cold operation explained here is the same method used successfully by MC Systems in the minus 55 Degrees C operation of wafer chucks and surface mount device handler systems. These systems exhibited temperature control stability of plus or minus 0.3 Degrees Centigrade at a control point of minus 55 Degrees Centigrade.

To operate in the Cold Mode, the Series 2000 Surface Mount Component Handlers or Model 8830 Environmental Chuck Systems(-55C to 155C) require that the user furnish a continuing source of chilled dry air or Nitrogen at a 4 to 5 liter per minute flow . This is usually Nitrogen Gas because of the dryness required if one uses air. This App Note will assume the use of Liquid Nitrogen and Nitrogen Gas. MC Systems Drawing 200090 shows in block diagram a practical method of tying everything together, however, there are several alternatives to this scheme. We chose LN2 and N2 for several reasons, availability, moisture free, environment friendly, minus 55 Degrees Centigrade easily attainable. Etc. As a transfer medium, you must use either dry air or Nitrogen Gas. To chill either dry air or N2 gas, one could use ice, dry ice, mechanical refrigeration, commercial liquid chillers, or any method capable of chilling a sufficient flow of either N2 or dry air to a temperature slightly below the desired control point.

Liquid Nitrogen Dewar Part Number LD35 is manufactured by Taylor Wharton of Indianapolis, IN. Taylor Wharton manufactures Dewars in several different sizes ranging from 4 liters to 50 liters. There is a liquid withdrawal device Part Number LWD installed on the LD35. The LWD is usually used either to withdraw LN2 from the Dewar or it may be used to refill the Dewar. In our system, the chilled gas (not liquid) is taken from the vent to the device handler or wafer chuck and is used to chill the DUT. The input to the Dewar is connected to the 5 to 8 PSI source of dry air or nitrogen gas. The Model LWD Liquid Withdrawal Device has a primary relief valve set at 10 PSI and a secondary relief valve set at 15 PSI. The N2 will then bubble up through the Liquid Nitrogen become a very cold gas and escape out of the vent into the handler thereby chilling the rail or chuck. The Temperature Control then heats the rail or chuck and controls at the desired temperature.

Obviously, any LN2 container with a stem for withdrawal might be used. A word of caution, not all LN2 containers are equipped with a stem and remember, DO NOT use the liquid withdrawal stem to supply N2 to the handler, USE THE GAS VENT to supply N2 to the handler.