Paper VT-MoM5
Creating a Controlled Gas Environment for Lifetime Testing of EUV Optics

Monday, October 21, 2019, 9:40 am, Room A213

Optics used in Extreme ultra-violet (EUV) lithography typically operate in an environment of 0.01 to 0.1 mbar hydrogen. Since EUV machines cannot receive a bake out after installation it is difficult to reduce background outgassing such as for water, oxygen, nitrogen and hydrocarbons. Although the partial pressures of these contaminants are orders of magnitude lower, their presence in combination with EUV irradiation induces oxidation, etching, deposition and other processes that affect the lifetime of EUV optics.

At TNO these processes are studied using an EUV beam line facility (EBL2). To enable this type of research a method was developed to create a well-controlled environment of multiple gases with defined and stable partial pressures.

The gas environment typically consists of hydrogen (10^-2 to 10^-1 mbar) with added oxygen, water, nitrogen and/or hydrocarbons (10^-8 to 10^-4 mbar). The strategy for controlling the gas environment is to start by setting the pressures of the additives prior to adding hydrogen. For accuracy, the pressure values are chosen such that the pump speed drop after adding hydrogen is taken into account. In order to do this, the pump speed of all gases needs to be known for both (ultra) high vacuum conditions as well as medium vacuum conditions (after addition of hydrogen). The procedure for setting the gas environment comprises 4 steps:

1. Determine the pump speed at HV conditions of relevant additives such as oxygen, water, nitrogen and/or hydrocarbons
2. Calibrate the differentially pumped RGA system with a gas mixture of hydrogen with defined, small fractions of additives.
3. Inject this mixture in the exposure chamber at nominal operating conditions. Using the RGA calibration data, the individual pump speed of each species in hydrogen is now calculated.
4. Set the partial pressures of additives prior to adding hydrogen, taking into account the calculated drop of pump speed.

Once step 1 to 3 have been carried out for all additives the resulting pump speed values can be applied for each exposure as long as the vacuum system and its geometry are not altered. As a result the partial pressures can be set quickly using common ion gauges.

The method developed to create a well-controlled environment of multiple gases along with its corresponding procedures and results will be presented.