Specification Cleanliness Proposal for 450mm Containers
By David Cheung, R&D engineer, Entegris Cleaning Process (Montpellier, France)
In the framework of the European R&D project E450EDL (European 450mm Equipment Demo Line)(1), ECP (Entegris Cleaning Process) collaborated with CEA-Leti and imec, to prepare for the 450mm transition. Many tests and evaluations have been performed regarding the use of 450mm wafers, based on the Flying Wafer®(2) concept. This article addresses the container specifications for 450mm wafers and the expected cleanliness levels for particle, metallic and airborne molecular contaminations for 450mm containers.
Experimental methodology linked to the Flying Wafer concept
A scheme for the Flying Wafer concept is presented in Figure 1, which shows the type of logistical loop that was implemented by the project partners for the pilot line. For instance, dedicated containers (MAC) were selected for the various tests and measurements and shipped from one location to another.
The containers were left open for several days in grey and clean rooms to allow collecting significant amounts of airborne contaminants from the environment (particles, metals, and molecular contamination). That enabled the containers to be considered in a "dirty" state. Then, a cleaning step was performed in a dedicated tool with adapted chamber size in order to evaluate the cleaning efficacy of 450mm containers, but also to estimate the potential transfer of various contaminations from the container to the wafers. Besides, appropriate measurement techniques and protocols have been developed according to the state of contamination and the type of contaminant to be measured. For each type of chemical contaminants, two different characterization methods were implemented to report the contamination of the containers as well as the contamination effectively transferred to stored wafers.
Particle contamination. The protocol is based on state-of-the-art LPC, adapted to 450mm containers ─ per SEMI E158-0314 Front Opening Unified Pod (FOUP) and SEMI E159-0314 Multiple Application Carrier (MAC). The measurement method consists of an aqueous leaching of the container. The container is shaken manually to soak all the inner surfaces, followed by a direct counting of the solution inside the container, via the dedicated optical counter.
Metallic contamination. The container’s contamination method is based on the metal contamination sampling by a leaching protocol with de-ionized water (DIW) of the inner surfaces of the 450mm container. The sampled liquid was then analyzed by Inductively Coupled Plasma Mass Spectrometry (ICPMS) for metallic impurities detection. Regarding the metals transfer to 450mm wafers, a specific test protocol, based on the storage and transport of clean 450mm wafers was defined. The 450mm wafers have been analyzed by Vapor Phase Decomposition (VPD)-ICPMS to quantify the amount of metallic impurities deposited on the wafer.
Volatile acids / ionic contamination. Ionic contaminants of the containers are evaluated by leaching with DIW followed by Ion Chromatography analysis (leaching-IC). The transfer of ionic contaminants as volatile acids to a stored wafer was performed through the exposure of a Cu coated 200mm Si wafer. This surface was selected since its great affinity to critical volatile acids. Then, the exposed wafer is analyzed by Liquid Phase Extraction (LPE)-IC to quantify the ionic contaminants.
Airborne organic contamination. Airborne organic contaminants inside containers are sampled with a Tenax® sorbent in passive mode. The tube is placed inside the container for seven days, the sampling flow ensured by molecular diffusion. Then, the sampling tube is analyzed by Thermal Desorption Gas Chromatography Mass Spectrometry (TD-GCMS), allowing the release and the quantification of trapped molecules. Those molecules were classified in three categories, depending on their volatility, such as:
- Volatile organics, as compounds between C5-C10
- Low volatile organics, as compounds between C10-C16
- Very low volatile organics, as compounds higher than C16
Furthermore, the transfer of organics from a 450mm container to a stored wafer was determined based on a 7-day storage on a cleaned 200mm silicon wafer. Then, the airborne organic contaminants deposited on this Silicon (Si) substrate are thermally desorbed, leading the released molecules to be analyzed by TD-GCMS, and allowing them to quantify the contaminant deposited in the rate of carbon atoms.
Results and proposal for 450mm container specifications
Following the protocol adopted for the Liquid Particle Count (LPC) measurement and according to the results obtained from cleaned versus non-cleaned containers, the specification thresholds for particle contamination cleanliness of 450mm containers can be set (see Table 1).
For metallic contamination, the proposed specifications for 450mm containers cleanliness are expressed in Table 2, based on the measurement method implemented for the project. In addition, the specification thresholds recommended for 300mm Flying Wafer® project are mentioned as well for comparison purposes. These levels have been defined to ensure that the use of 450mm containers fulfils the stringent International Technology Roadmap for Semiconductors (ITRS) wafer metallic contamination specifications.
For ionic contamination, the proposed specifications for 450mm container cleanliness are shown in Table 3, based on the measurement method used for the project. These container specifications have been developed to control the surrounding wafer environment in accordance with the wafer ITRS specifications and from the database obtained from various 450mm containers.
The proposed specifications for organic contamination for 450mm container cleanliness are shown in Table 4. These container specifications are based on the measurement method used for the project on containers as well as on wafers stored in the containers for cross-contamination evaluation. They have been developed to align with the wafer total organic contamination recommended by ITRS. Moreover, a specification related to lower volatile compounds has been proposed both in the container ambient and at the stored wafer level.
We have been able to define the specification thresholds to be set up in terms of cleanliness (particle, metallic, ionic and organic contaminations) for 450mm containers. This document addresses the main recommendations for the safe use of 450mm containers in terms of cleanliness to avoid any risk of cross-contamination from the containers to the wafers stored inside.
- With the support of Eniac-JU program and public authorities from participating countries
- Flying Wafer® is a name registered by the Fraunhofer-Gesellschaft zur Fördering der angewandten Forschung e.V. in the Register of Community Trade Marks
David CHEUNG is R&D engineer at ECP and works as an expert in cleaning and contamination control. He has been managing the R&D programs and collaborative European projects for more than 10 years. His main objective is to develop advanced cleaning processes addressed to sensitive devices, associated to the most appropriate characterization means.
July 20, 2016