A new class of adsorbent materials offer high capacity storage and safe delivery of dopant gases
Jose Arno, Omar Farha, William Morris, Paul Siu, Glenn Tom, Mitch Weston, and Patrick Fuller,
NuMat Technologies, Skokie, IL, USA
Justin McCabe, and Mike Ameen, Axcelis Technologies, Beverly, MA, USA
Solid State Technology April/May 2018 Issue
Metal-Organic Framework (MOF) materials are a new class of crystalline adsorbents with broad applicability in electronics materials storage, delivery, purification, and abatement. The adsorbents have unprecedented surface areas and uniform pore sizes that can be precisely customized to the specific properties of electronic gases. ION-X is a sub-atmospheric dopant gas delivery system designed for ion implantation, and the first commercial product that uses MOFs. The performance of ION-X delivering arsine, phosphine, and boron trifluoride was evaluated at Axcelis and compared to the incumbent products. In-process and on-wafer performance of the MOF-based dopant gases compared positively to conventional source gases. Flow, pressure, and beam stability were undistinguishable from conventional gas sources throughout the lifetime of the cylinder. Beam and wafer contamination levels (both surface and energetic) were below specification limits, matching the performance of the reference qualified product.
Dopant Gas Safety Challenges
The storage and delivery of hazardous gases creates significant environmental, health, and safety challenges. Their usage requires implementation of stringent safety control systems to minimize the risks of exposure to humans and the environment. The dangers associated with handling toxic gases are the result of both the inherent chemical hazard of the molecule and the kinetic energy stored in the vessel in the form of compression. In essence, the lethality of a toxic release is magnified exponentially by the energetic force of the high-pressure storage. Historically, one way to mitigate these risks was to dilute the hazardous material with inert gases in an effort to attenuate the toxicity effects. Depending on the concentration, this solution provides a safety factor improvement of 10 or 100 by virtue of reducing the molecular density of the hazardous gas to 10% or 1% mixtures, respectively. This approach is commonly used in the electronics manufacturing industry for gases that are known to have extreme toxicity. Hydride gases (i.e. arsine, phosphine, germane, or diborane) are examples of such highly toxic gases used as dopant sources in a number of electronic manufacturing processes. While this dilution method is effective at reducing the toxicity levels, these mixtures are typically produced at cylinder pressures significantly higher than the pressures of the pure toxic gases. In a release event, this solution reduces the lethality of the dose at the expense of a higher release rate.
In 1993, ATMI (now an Entegris company) introduced a different approach to reduce the toxic gas storage hazards  . The technology involves using nano-porous adsorbents to condense the gas molecules onto their surfaces. This process effectively reduces the kinetic energy of the gas, thus reducing the pressure in the gas cylinder. The large available surface areas within these materials result in gas storage capacities comparable to the high-pressure cylinders. The intrinsic safety advantages of adsorbed gas cylinders are derived from the reduction in pressure within the cylinder. Typically, these vessels are filled to sub-atmospheric pressures (measured at room temperature) in order to inhibit an outward gas release in the event of a leak.
Figure 1. Structure of a representative MOF material at different scales
The first sub-atmospheric dopant gas delivery systems used zeolites (SDS® 1) while the second and third generations (SDS® 2 and SDS® 3) evolved to activated carbon adsorbent materials. These gas cylinders store and deliver dopant precursor gases (primarily arsine, phosphine, and boron trifuoride) predominantly for ion implantation processes. In its third generation, and in order to further improve gas storage capacities, SDS® 3 evolved by creating a highly dense monolithic adsorbent that nearly eliminated void volumes in the cylinder.
In this paper, we describe a new sub-atmospheric gas delivery system (ION-X) that uses a novel ultra-high surface area class of materials called metal-organic frameworks (MOFs). In addition, the implant process performance using the new product delivering arsine, phosphine, and boron trifluoride was evaluated in a major ion implant OEM facility will be described.
MOF Overview: The Next Generation in Nano-Porous Adsorbents
MOF are three-dimensional crystalline structures assembled with metal-containing nodes connected by organic links (Figure 1). The resulting highly organized molecular structures generate nano-pores with unprecedented surface areas [2-4]. In addition, the large number of available metal nodes and organic linkers provide unparalleled molecular design flexibility to tailor the chemical and physical properties of the adsorbent material to fit the application. Since their discovery in the early 1990’s, MOFs evolved from an academic curiosity to a widely recognized new class of materials with endless practical applications in energy, specialty chemicals, military, medical, pharmaceutical, and electronics industries. Not surprisingly, MOFs are one of the fastest growing classes of materials, with thousands of experimental structures now being reported.
For gas storage and delivery applications, MOFs’ design flexibility provides significant advantages over traditional adsorbents (Figure 2). Namely, the pore size, surface area, and chemical stability can be tailored to the specific properties of the adsorbed gases. Compared to zeolites and activated carbon adsorbents, MOFs have significantly larger surface areas (up to 7,000 m2/g has been reported). This property, combined with bulk density, is critical in gas storage applications where capacity is measured in terms of vessel volume rather than adsorbent mass. Pore size tunability is also an important parameter in efforts to match the dimensions of the MOF cavities to the molecular sizes of the target adsorbates. This parameter impacts adsorption capacities (how much gas can be loaded) and desorption characteristics (how much can be delivered as a function of pressure).
Figure 2. Properties of selected adsorbents
Unlike the broad pore size distributions found in activated carbon adsorbents, MOFs’ crystallinity results in more “usable” pores. This pore size uniformity also results in higher gas quality, as impurities are selectively size excluded.
Preventing reactions between the adsorbent and the target gas is extremely important in electronics applications. Adsorbent/gas interactions will contribute to gas decomposition, leading to impurities and unwanted dopant gas composition changes that could affect the process. The molecular composition of zeolites and carbon adsorbents are limited to a few elements (typically carbon, aluminum, and silicon) and their oxides. MOFs, on the other hand, can be synthetized from a near-endless range of organic and inorganic constituents, offering more options for creating stable gas/adsorbent interactions.
MOF-based Gas Delivery System for Ion Implant gases
ION-X (Figure 3) is a sub-atmospheric dopant gas storage and delivery system designed for ion implantation . This new product is commercially available through a distribution partnership between NuMat Technologies and Versum Materials. ION-X uses individual MOF structures with tailored pore sizes to effectively and reversibly adsorb arsine, phosphine, and boron trifluoride gases. The pressure in filled ION-X cylinders is below one atmosphere, significantly reducing the health and environmental impact of an accidental gas release. Furthermore, MOFs’ ultra-high surface areas and uniform structures provide capacity and deliverable advantages compared to existing carbon adsorbent-based products (Figure 4). It is important to note that the first-generation ION-X cylinders utilize granulated MOFs with similar adsorbent bulk density to the first-generation carbon product: for the same mass of adsorbent, MOFs provide 40% to 55% higher gas delivery by virtue of their superior surface area and pose size uniformity. Analogous to the evolution of SDS® 2, MOF densification inside the cylinder will further increase the gas capacity in next-generation ION-X products.
Figure 3. ION-X 2.4 L dopant gas cylinder
Implant Performance Characterization
The performances of hydride and BF3 ION-X dopant delivery systems were recently evaluated by Axcelis Technologies. The test plan included flow, mass spectral, and metal contamination analyses (both at the surface and at implanted depth). The evaluations were performed using a Purion H 300 mm high current ion implanter installed at Axcelis’ Advanced Technology Center (Beverly, MA, USA). The experiments were repeated using commercially available and well-established sub-atmospheric dopant gas sources in order to provide a basis for comparison.
Cylinder installation and setup was seamless, requiring no modifications to the existing gas box hardware or software. Flow rate stability for all three gases (AsH3, PH3, and BF3) was demonstrated in the 3.5 to 8 sccm ranges down to cylinder pressures of 20 torr (spec limit). For arsine, the flow experiment continued through a full cylinder depletion, showing a stable flow rate down to cylinder pressure below 3 torr.
Figure 4. Dopant gas delivery comparison (down to 20 torr) between carbon-based incumbent products and ION-X
The beam energy, purity, and stability were evaluated by analyzing the mass spectra generated during the implantation processes. In all cases, the target dose was set to 5 x 1015 at/cm2 with beam energies of 40 keV, 20 keV and 15 keV for As+, P+, and BF2+ ion implants respectively. The stability and purity of the target doping ion beams were within specifications and very similar to the ones produced by the reference gas sources. Based on the mass spectra, ION-X did not generate any impurities derived from either gas or MOF decomposition.
Neutral and energetic metal contamination levels were thoroughly investigated in this study. All metal analyses were performed by sampling wafers produced using the recipes described in the previous paragraph. Vapor Phase Decomposition-inductively coupled Plasma-Mass Spectrometry (VPD-ICP-MS) was used to monitor the contamination from key trace metals at the wafer surface. Particular attention was placed on monitoring zinc and iron, as these are the metals used in the hydride and BF3 ION-X MOF adsorbents respectively. Results show that all metal levels were within below specification limits and compared well to the levels detected in control wafers. In all cases, zinc and iron surface contamination levels were below their corresponding detection limits of 0.03 and 0.05 x 1010 atoms/cm2.
Energetic metal contamination is of special interest in ion implantation as even low levels of impurities could affect the performance of the electronic devices. The depth profile of the metals used in ION-X’s MOFs composition were measured using Secondary Ion Mass Spectrometry (SIMS). Wafers used for SIMs analyses were doped using both ION-X and incumbent gas sources using the same ion implant tool and previously stated recipes. The zinc and iron metal concentration profiles for the hydride and boron implants were well within specifications and show no discernable differences between the incumbent and the MOF-based gas sources (Figure 5). These results, combined with the previous surface contamination tests, conclusively establish the gas and ion purity of the dopant species extracted from ION-X adsorbents. Moreover, the results are consistent with extensive gas analyses performed at NuMat after subjecting the MOF adsorbent materials to accelerated aging, vibration, and cycle testing.
Figure 5. Comparison of energetic Zn (hydrides) and Fe (BF3) levels using incumbent and ION-X gas sources
This article provides process and on-wafer performance of ION-X, a new MOF-based dopant gas delivery system. The adsorbents used in these cylinders have unsurpassed surface areas, stability, purity, and pore sizes ideal for the storage and delivery of ion implant dopant gases. In-process and on-wafer performance of boron trifluoride, arsine, and phosphine dopant sources compared positively to conventional source gas cylinders. The issue of contamination was investigated in detail, demonstrating that the new adsorbents do not contribute to surface or energetic metal impurities. The results published in this article provide independent evaluation of the new product, supporting the safe use of this product in mainstream ion implant applications. To that end, ION-X is already qualified and being used at an electronics manufacturing site with confirmed high stability and purity performance.
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