Haze formation on the patterned metal surface of reticles is a known problem for IC manufacturers that can impact device yield and increase operational costs due to the need for more frequent cleaning of the reticles. Storage of reticles in an ultraclean environment can reduce haze formation and reduce operational costs.
We examined the contamination levels of a new type of reticle stocker that stores reticles in microenvironments which are continuously purged with extreme clean dry air (XCDA). Each microenvironment consists of twelve vertically stacked reticle storage slots which can be opened at any slot. The design of the microenvironment includes an XCDA supply that provides a homogeneous horizontal flow of XCDA between the reticles.
Figure 1. Reduction of contamination levels inside the storage microenvironment as a function of XCDA flow rate.
As shown in Fig. 1, continuous XCDA purge reduces the contaminant levels inside the microenvironment. The amount of reduction depends on the XCDA purge flow rate and the chemical species. Volatile organic substances can be reduced by more than two orders of magnitude. Humidity is reduced less because the plastic material of the storage microenvironment incorporates water in its matrix and can release moisture to the extremely dry atmosphere. Chemical filters applied to mini- or microenvironments typically reduce the contaminant levels only by 95-99% and do not reduce the humidity.
To pick and place reticles, the reticle storage microenvironment must be opened. The transient contaminant levels inside the empty microenvironment show an increase at the moment when the microenvironment is opened. Under the given conditions, the microenvironment returns to equilibrium levels with a time constant of 105 seconds (see Fig. 2). Similar dynamic response was measured for IPA and acetone.
Figure 2. Transient humidity when the storage microenvironment was opened for reticle handling.
The impact of handling on reticles stored inside the microenvironment was found to depend on several factors. When the microenvironment is filled with reticles, the horizontal XCDA flow purges a smaller volume of air and therefore re-establishes the equilibrium condition more quickly than in an empty microenvironment. The recovery time constant of a filled microenvironment was measured as only 20 seconds compared with 105 seconds for an empty microenvironment. Reticles below and above the opened slot were found to act as "shield" for the rest of the microenvironment and reduce the impact of opening the microenvironment. Moreover, the impact depends on the distance between the storage position and the opened slot: the larger the distance the smaller the impact. The last two factors
reduced the peak value in the transient humidity measurements to one third when the microenvironment was opened 5 slots away from the storage slot.
GLOBALFOUNDRIES Fab 1 used the results of these studies to optimize the parameter settings for its reticle storage. The reticles are stored in microenvironments which utilize extremely low contaminant levels. Based on the current utilization profile of the reticle stocker, the storage microenvironments are opened for approximately 0.01% of the time the reticles are stored in the stocker. Even in the short opening times the contamination levels do not exceed the ITRS requirements for reticle storage (see ITRS 2011, YE3 limits for reticle pod interior, supporting ≤ 28 nm technology nodes).