Front Opening Unified Pod Research Articles

Dynamic boundary conditions for a coupled convection-diffusion model with heat effects: applications in cross-contamination control

This work investigates the mass transfer of the Airborne Molecular cross Contamination (AMCs) between the Front Opening Unified Pod (FOUP) and wafer (silicon substrates) during the microelectronics devices manufacturing using dynamic boundary conditions. Such cross-contamination phenomena lead to detrimental impact on production yield in microelectronic industry and a predictive approach using modelling and computational methods is a very strong way to understand and qualify the AMCs cross contamination processes. The FOUP is made of polymeric materials and it is considered as a heterogeneous porous media, it can adsorb and desorb the contaminant, thus the modelled processes are the contamination of two-component in transient flow. Coupled diffusion and convection-diffusion model with heat effects are used to define the phenomena. The present methodology is, first using the optimization methods with the numerical solution in order to define the physical constants of various materials which have been studied experimentally and separately, and the second using the finite element methods including these physical constants and relevant interface condition in order to take into account the adsorption kinetics law. Numerical methods to solve the problem are proposed. The dynamics behaviour of the AMCs analysis was determined thanks to the switch of Dirichlet to Neumann condition. The mathematical model preserves the classical forms of the diffusion and convection diffusion equations and yields to consistent form of the Fick's law. The computed results are in correlation with the experimental measurements. Some numerical results are presented in this work.

Evaluation of Hindrance to the Growth of SiN Passivation Layer by Contamination of Fluoride Ions in Front Opening Unified Pod (FOUP)

We have investigated the hindrance to the deposition growth of silicon nitride (SiN) passivation layer from the contamination by airborne molecules in the front opening unified pod (FOUP). In particular, an artificial contamination of FOUP by fluoride ions as the source of the contaminants is utilized to elucidate the influence of contamination on the wafer surface. When the bare wafer surface is exposed to fluoride ions in the contaminated FOUP, the deposited thickness of the SiN layer is observed to decrease to a maximum of 11 A from our experimental condition. On the other hand, there is no appreciable variation in the thickness of deposited SiN layer stored in the pre-cleaned FOUP. Based on the analytical results of wafer surfaces and FOUPs, we believe that the contamination of fluoride ions on wafer surfaces is originated from the contaminated surface of FOUP. Therefore, we conclude that it is necessary to clean and monitor the inside of FOUP on a regular basis, especially after wet or dry etching processes, which generates gaseous impurities.

Control of HF Volatile Contamination in FOUP Environment by Advanced Polymers and Clean Gas Purge

Semiconductor manufacturing technologies have developed to the point where molecules, such as water, oxygen and airborne molecular contaminants (AMCs), have become detrimental in specific process conditions. Front Opening Unified Pods (FOUPs) are designed as controlled microenvironments (MEs) that protect processed wafers from AMCs during storage and transport. However, it has been demonstrated that FOUPs are able to accumulate by sorption molecules outgassed by processed wafers. Such contaminants are then able to be subsequently released and transferred to other sensitive wafers leading to detrimental impact [1,2]. This cross-contamination scheme from FOUP to wafer was evidenced especially for volatile acids such as HF or HCl and is responsible of yield losses due to drastic corrosion issues or crystal growth on Cu, Al or TiN materials [1,3,4]. These cross contamination issues can be reduced or controlled using low sorption and outgassing polymer materials as previously reported [4,5]. Another AMC control measure is to purge the FOUP with a dry gas. This provides several advantages, for example, wafers are not easily oxidized thus preventing oxide layers, deposition of hydrocarbons and metal defects [6] However, there is no measured information about the purge impact has on AMC control inside a FOUP. The purpose of this paper is to show and quantify what effect two different FOUP polymers using nitrogen and clean dry air (CDA) purge have on the HF volatile acid control through the Cu-wafers storage.

FOUP Material Influence on HF Contamination during Queue-Time

Airborne Molecular Contamination (AMC) concentrations become critical during queue-time (between two successive process steps) when wafers are degassing inside Front Opening Unified Pod (FOUP), a confined environment [1]. In that case, AMC concentrations can reach maximum level.

Use of a Purged FOUP to Improve H-Terminated Silicon Surface Stability Prior to Epitaxial Growth

A H-terminated Si surface serves as the starting surface for hetero-epitaxial growth. The presence of oxygen on the wafer surface caused by surface re-oxidation can significantly reduce the epitaxial quality. In this paper we evaluated the capabilities of an Entegris EBM (Entegris Barrier Material) purged FOUP (Front-Opening-Unified-Pod) to reduce this re-oxidation. The surface cleanliness is one of the factors affecting (hetero-) epitaxial quality. Clean starting surfaces can be obtained by a high temperature H2-bake (T > 800 °C). [1] Surface reflow during these high temperature bakes can significantly impact the integrity of small structures, e.g. causing corner rounding of FINFET structures. [2] As the clean starting surface can no longer be obtained by the H2-bake at relative high temperatures, more stringent requirements are put on the pre-epi clean and on the impact of queue time between wet-chemical clean and epitaxial growth. We have evaluated the re-oxidation of H-terminated 300 mm Si surfaces in a non-purged FOUP compared to an EBM purged FOUP while the FOUP is open and docked to the factory interface.Re-oxidation or degradation of the H-terminated Si surface has been evaluated by capping the surface with an epitaxial-grown 80nm Si-cap/80nm Si0.8Ge0.2 bilayer. The H-terminated surface was created in a separate step before the epitaxial growth by applying a thermal treatment in H2 in a conventional ASM Epsilon3200 epi reactor. The Si wafer was unloaded in between the thermal treatment and the Si-cap/SiGe epitaxial growth step. This procedure offers the most-controllable scheme for creating a H-passivated surface but the learnings are also applicable to wet-chemical pre-epi treatments. The Si0.8Ge0.2 / Si substrate interfacial cleanliness was evaluated at the center of the wafer by SIMS (Fig. 1). The EBM purged FOUP shows a lower interfacial degradation compared to the standard FOUP. The uniformity of the surface degradation has been investigated by surface light scattering (data not shown). While the light-scattering intensity and its range increased significantly after queuing for 1hr in a non-purged FOUP, this increase was suppressed when using the N2-purged FOUP. The light scattering measurements also show that the epitaxial quality is better in a N2-purged FOUP compared to a non-purged FOUP but degraded compared to the loadlock reference.In this paper we evaluated the impact of queue time of an H-terminated Si surface in a non-purged FOUP compared to an EBM N2-purged FOUP. The interfacial degradation as measured by light scattering and SIMS for 1hr of queue time is shown to be less when the EBM N2-purged FOUP is used. However the epitaxial quality degradation was shown not to be completely suppressed.

Characterization of Mass Transfer Rates and Contamination Kinetics on Silicon Wafer Surface

Production yield in semiconductor wafer fabrication is directly affected by low levels of molecular contamination of the silicon wafer surface. Wafers are often carried in specialized plastic enclosures, called front opening unified pods, that are continuously purged with a nitrogen flow to minimize wafer surface contamination. To improve this purge process, it is necessary to better understand the effect of mass transfer transport and kinetic processes on the silicon wafer surfaces. The experimental surface kinetics data available in the literature for diethyl phthalate were utilized, along with a validated computational fluid dynamics model to predict the relative magnitude of the time scales associated with transport and kinetics. The transport time was found to be considerably shorter than the characteristic adsorption time, and the desorption characteristic time was longer than the adsorption time. In general, surface kinetics parameters are not always known. Among other techniques, optimization techniques can be employed to calibrate the kinetic rate parameters. A multi-hierarchical model optimization technique can then be used to infer surface kinetics rates using experimentally measured concentration data for amine contamination of a wafer surface.

Heuristic scheduling policies for a semiconductor wafer fabrication facility: minimizing variation of cycle times

This paper presents heuristic scheduling policies for semiconductor wafer fabrication facilities. The proposed heuristic scheduling policies include the advanced operation due date (OPNDD) for a sequence control policy and the adaptive constant work-in-process (CONWIP) for an input release control policy. The objective of the proposed scheduling policies is to reduce the variation of cycle times in the wafer fab. The advanced OPNDD sets the higher priority to the front opening unified pod (FOUP) with the smallest operation due date that is computed using a generalized stochastic Petri net model, and at the same time regulates the queue lengths of the FOUPs in each stoker by preventing excessive queue lengths in bottleneck workstations. The adaptive CONWIP controls dynamically the input release time of FOUPs using the adaptive WIP level according to the current status of the wafer fab. The simulation experiments show that the proposed scheduling method is efficient in reducing the variation of cycle times.

A comparative study of the HF sorption and outgassing ability of different Entegris FOUP platforms and materials

Front Opening Unified Pod (FOUP) polymers are able to strongly attract airborne molecular contaminants (AMC) coming from the connection to equipment or from the release of just processed wafers but will also later outgas these contaminants. In consequence this subsequent outgassing of species trapped in plastic containers constitutes an important issue to wafer environmental contamination control. In this work, a comparative study of different Entegris FOUP platforms and materials was carried out in order to quantify their capacity to be contaminated by HF through intentional contamination. The subsequent release of this molecule was quantified in terms of outgassing, leaching and transfer over AlCu-wafers. Results obtained allowed us to establish the potential risk in terms of defectivity probability on wafers and to classify the potential cross contamination from FOUP to wafer in three groups: low, medium and high.

Monitoring and Purge Parameters of Ammonia for the Front Opening Unified Pod (FOUP)

Monitoring and Purge Parameters of Ammonia for the Front Opening Unified Pod (FOUP)

Assessment of a FOUP Conditioning Equipment for Advanced Semiconductor Application

The effectiveness of a FOUP (Front Opening Unified Pod) conditioning test bench was tested with respect to the reduction of outgassing materials from FOUP materials and cleanroom compatibility. The equipment uses vacuum and heat to reduce volatile organic compounds from construction material and wafers. Five different analysis methods were used to asses the performance of the equipment with respect to organic, ionic and metal contamination. The experiments showed that the use of heat and vacuum has a positive effect on the organic contamination from FOUPs, and that the equipment is suitable for semiconductor applications in cleanroom environments.

Optimal Airflow Performance for FOUP Systems in Cleanrooms Using SVE Quantification Method

This research examined the possibility of optimizing airflow performance of cleanrooms with FOUP (Front Opening Unified Pod) systems used in 300mm wafer lines for VLSI production. Numerical simulations using computational fluid dynamics (CFD) were carried out to obtain airflow states in cleanrooms with various design conditions. Each result was evaluated using a new quantification method and the cleanroom performance was then compared to it. The proposed method was based on the idea of Scale for Ventilation Efficiency (SVE), which is used to measure building ventilation. It enabled quantitative evaluation of the airflow performance independent from other factors and could be used to help design optimal cleanrooms. The study found that cleanroom performance changes with varying supply types and filter coverage, and that there is a proper combination of these parameters. Therefore, it is possible to develop the most efficient FOUP cleanroom system using this method.

Multiple orders per job formation and release strategies in large-scale wafer fabs: a simulation study

In this paper, multiple orders per job type formation and release strategies are described for semiconductor wafer fabrication facilities (wafer fabs). Different orders are grouped into one job because orders of an individual customer very often fill only a portion of a Front-Opening Unified Pod (FOUP). After job formation, an FOUP is assigned to each job and is used to move the job throughout the wafer fab. We determine an acceptable number of FOUPs given a prescribed order release rate to find appropriate values for on-time delivery performance measures, cycle time, and throughput by discrete event simulation. On the other hand, given a prescribed number of FOUPs and target values for these performance measures, we look for an appropriate order release rate. Simulation experiments with models for large-scale wafer fabs are performed to solve these two problems. We also discuss a complementary analytical method to determine an appropriate number of FOUPs in some specific situations.

Integrated heuristics for scheduling multiple order jobs in a complex job shop

Scheduling in semiconductor manufacturing involves numerous types of complexities, including assignment of orders to front-opening unified pods (FOUPs), assignment of FOUPs to batches, and batch processing on single or parallel machines in multiple tool-groups with re-entrant flows. Based on these features, wafer fabrication in semiconductor manufacturing is referred to as a complex job shop in the literature. Assignment of multiple customer orders to jobs/FOUPs and scheduling them in a complex job shop environment is labelled as MOJ-CJSSP (multiple orders per job complex job shop scheduling problem). In this paper, we present constraint programming (CP), ant colony optimisation (ACO), and integrated CP-ACO approaches to minimise the sum of weighted completion times of the orders in MOJ-CJSSP.

A column generation heuristic for complex job shop multiple orders per job scheduling

Scheduling in semiconductor manufacturing is a challenging task due to inherent complexities like assignment of multiple customer orders to front opening unified pods (FOUPs), batch processing in multiple toolgroups with parallel machines, and re-entrant flow. A disjunctive–network-flow mixed integer program (DNF) and a column generation heuristic are presented to minimize the sum of weighted customer order completion times in a complex job shop environment. In addition, a lower bound estimation approach for the complex job shop problem is presented. The column generation heuristic obtains solutions that are very close to that of the time constrained DNF formulation for problem instances with zero order ready times. However, column generation’s performance differs considerably for problem instances with non-zero order ready times and is able to solve large problem instances in a matter of minutes.

Dynamic Analysis on Particle Concentration Induced by Opening the Door of a Front Opening Unified Pod (FOUP) That Loaded with 25 Pieces of 300 mm Wafer Manufacturing Processes

This study investigates experimentally and numerically the influences of FOUP door opening speed (Vdoor) and the pressure difference (dP) on the dynamic distributions of the averaged non-dimensional particle concentration in the FOUP (Cave), which is defined as averaged particle concentration in the FOUP (cave) divided by the particle concentration in the clean room (cCR). Here dP is the difference between the mini-environment pressure and the clean-room pressure. Results show that when Vdoor lies between 0.05 m/s~0.15 m/s and dP within 0.3 Pa-12.7 Pa, Cave proportional to 1/2ρVdoor2 and inversely proportional to dP. This can be expressed by a stepwise multiple regression equation: Cave = 4.56 × 10-1 (1/2ρVdoor2) – 4.6 × 10-4 dP + 4.96 × 10-3.

Dynamic Analysis on Particle Concentration Induced by Opening the Door of a Front Opening Unified Pod (FOUP) That Loaded with 25 Pieces of 300 mm Wafer Manufacturing Processes

This study investigates experimentally and numerically the influences of FOUP door opening speed (Vdoor) and the pressure difference (dP) on the dynamic distributions of the averaged non-dimensional particle concentration in the FOUP (Cave), which is defined as averaged particle concentration in the FOUP (cave) divided by the particle concentration in the clean room (cCR). Here dP is the difference between the mini-environment pressure and the clean-room pressure. Results show that when Vdoor lies between 0.05 m/s~0.15 m/s and dP within 0.3 Pa-12.7 Pa, Cave proportional to 1/2ρVdoor2 and inversely proportional to dP. This can be expressed by a stepwise multiple regression equation: Cave = 4.56 × 10-1 (1/2ρVdoor2) – 4.6 × 10-4 dP + 4.96 × 10-3.

Reduction of Airborne Molecular Contamination on 300 mm Front Opening Unified POD (FOUP) and Wafers Surface by Vacuum Technology

With decreasing critical size of micro-electronic fabrication imposed by ITRS today, Front Opening Unified Pod (FOUP) has been designed to transport the silicon wafers in the 300 mm semiconductor fab to decrease the particles contamination, without taking the Airborne Molecular Contamination (AMC) and moisture problems into account. Various methods of AMC decontamination methods has been introduced in past, such as purging mini-environment with nitrogen, however the efficiency of its yield improvement capacity has not been proven. An ultimate AMC decontamination method with vacuum decontamination and passivation technology shows a very good efficiency on the AMC removal mechanism, with a direct impact on the yield improvement.

Column generation heuristics for multiple machine, multiple orders per job scheduling problems

Front opening unified pods (FOUPs) are used to store and transport silicon wafers in 300-mm semiconductor wafer fabs. To achieve production efficiencies, wafers are grouped together in FOUPs without regard to the originating customer placing the order. In the resulting multiple orders per job (moj) scheduling problem, scheduling is performed at the FOUP (i.e., aggregated order) level, while scheduling performance is assessed per individual customer order. Column generation heuristics are presented for single and parallel machine moj scheduling problems to minimize total weighted order completion time. The proposed heuristics obtain near-optimal solutions very quickly, outperforming competing approaches in the literature.

Design and evaluation of a nitrogen purge system for the front opening unified pod (FOUP)

The technology roadmap toward smaller structures and thinner layers in semiconductor manufacturing directs attention more and more toward yield-affecting influences from the air quality of manufacturing environment. The so-called airborne molecular contaminants (AMCs) have become a key issue related to production yield. A useful method for reducing AMC is purging of wafer with nitrogen gas. In this study, parametric studies on purging a front opening unified pod (FOUP), a wafer box for handling 300-mm wafers, are studied by theoretical analysis, CFD simulation and experimental measurement. The results show that the inlet purge gas through plenum injector with 60° of diverging exhibits faster decrease in oxygen concentration than that with greater degree cases. The depletion on the 1st wafer is slow because of the uni-distance configuration of holes of the plenum injector, which created a non-uniform purge gas inlet profile. By adopting a ceramic plenum injector, a uniform inlet purge gas profile is created and the purge efficiency is therefore significantly improved.

Purging of Front-Opening Unified Pod with Nitrogen for 300 mm Wafer Manufacturing

The control of airborne molecular contamination (AMC) plays an increasing role in semiconductor manufacturing processes. We conducted a parametric study of purging a front-opening unified pod (FOUP), a wafer box for handling 300 mm wafers, with nitrogen experimentally, analytically and numerically. Factor considered to affect purging include the coefficient of absorption and desorption of water vapor through the FOUP's polycarbonate material, the configuration of plenum injector, the best composition of plenum injector and the FOUP, and the selection on purge flow rate. This study clarified purge phenomena.