Metrology-aided gas purification development
ABNEESH SRIVASTAVA, THOMAS GAFFNE, Entegris, Inc. San Diego, CA.
An enhancement in contaminant moisture removal is achieved for inert (nitrogen, noble) and hydrogen gas purifiers by tailoring purifier adsorbent properties.
Gases like nitrogen, hydrogen and argon are routinely used in the semiconductor industry for tool purge, precursor carrier, plasma-assisted growth, in situ cleaning and annealing for both epitaxial processes such as MOCVD, MBE and lithography processes such as immersion and EUV imprinting for fabrication of nano-structured devices for LEDs, lasers, HEMT and memory ICs. These technologies involve a wide gamut of process variables including pressure, temperature, substrate, laser irradiation, and reaction chemistry. It is therefore desirable to mitigate extraneous chemical and physical phenomena that can limit device optical and electrical performance. Elimination of water vapor, a key contaminant, from MOCVD processes is necessitated to control doping and growth rate critical to epilayer morphology. Degradation of photoluminescence in LED, laser based optical devices has been extensively co-related to presence of oxygen-containing impurities.
An increased use of hydrogen gas with the emergence of EUV lithography for mirror purging and contamination removal is anticipated. These place stringent demands on contamination metrics for use of hydrogen gas in vacuum based applications. Additionally, at smaller wavelengths, as in EUV (13.5 nm), relatively higher photoabsorption cross-sections would elicit low carbon and oxygen contamination levels in order to minimize damage to lithographic optics. Oxygen from dissociated water molecules can oxidize the mirror surface resulting in loss in reflectivity.
Typically gas dependent processes start with 5-6 N grade purity levels and employ gas purification technology to achieve 9-10 N grade gas purity. This translates to removal of 1-10 ppmv (parts per million by volume) contamination (impurity) challenge at inlet to produce 10-100 pptv purity outlet levels. Targets of high throughput and low defect rates for semiconductor devices translate into higher contaminant retention capacity and removal efficiency for the gas purification industry. Added demands of an ambient temperature regenerable purification media and removal of a broad contaminant group to maximize COO requires a detailed probe of the purifier media-contaminant interaction at the molecular level under trace conditions.
In this article we provide an example of the symbiotic relationship between metrology and gas purification development. A state-of-the-art mass spectrometric based analyzer, APIMS, is used to both track and discover material properties relevant to efficient moisture removal from inert gases. A comparison of drydown response, moisture capacity and removal efficiency are critical metrics used to evaluate gas purification performance. A description of these three and a fourth metric to stress purifier purity stability is provided after a brief description of the test method.
Quadrupole based Atmospheric Pressure Ionization Mass Spectrometry (APIMS) is the research and industrial standard for on-line detection of a wide range of gaseous molecules at sub-100 pptv levels in real time. APIMS serves as an important tool for establishing removal efficiency, lifetime capacity, co-contaminant levels in gas purification. Combination of chemical ionization under atmospheric conditions and mass spectrometry allows for high sensitivity for a wide range of contaminants.
The test apparatus consists of a moisture source and a purifier test manifold connected to the inlet of APIMS (Extrel CMS, LLC). The moisture source manifold is used to generate a moisture standard in dry N2 gas for APIMS calibration and purifier test challenge gas. A constant flow through the purifier is maintained during the test and sent to the APIMS inlet. All tubings are electropolished 316L stainless steel and the API source is operated under optimized conditions. Best gas manifold practices are adopted to minimize moisture outgassing. Moisture source concentrations are continuously monitored using a commercial cavity-ringdown spectrometer (Laser Trace, Tiger Optics).
Figure 1. The prominent peak is from the bulk nitrogen carrier gas. A smaller N+ fragment peak is also observed.
Moisture signal in the APIMS mass spectrum is observed at the parent molecular mass ion, H2O+ mass-to-charge, m/z =18 (Fig. 1). The prominent peak is from the bulk nitrogen carrier gas, at parent mass ion, N2+ m/z = 28. A smaller N+ fragment peak is observed at m/z=14. Formation of m/z=18 ion involves a charge transfer reaction between the ionized bulk carrier gas molecule, N2 (reaction 1) and neutral impurity gas molecule, H2O. The charge transfer reaction (reaction 2) is facilitated by the lower ionization potential(IP) of the impurity gas molecule, H2O (IPH2O = 12.60 eV) in comparison to the bulk carrier gas, N2 (IPN2 = 15.58 eV).
Drydown response, "upset" recovery
Typically when a component is installed in a process line there is an associated "upset" recovery time needed for moisture drydown causing to tool downtime. A recovery upset test was conducted per SEMI F58-1000 on Entegris Gatekeeper purifier filled and activated with H and HX gas purification medias. Entegris, Inc. introduced HX gas purification material for inert (nitrogen, noble) and hydrogen gas contamination removal to replace H gas purification material. An initial moisture level of < 300 pptv, (Fig. 2), was established before replacement of spool piece with the test purifier. During purifier insertion the gas flow was diverted to the APIMS via a bypass line. A moisture spike of ~ 5 ppbv (parts per billion by volume) is observed on establishing the gas flow through the purifier line. Levels of <300 pptv are re-established in less than half the time for the HX purifier. Over a 10-hour time period both purifiers exhibit similar drydown rates with an overall two-fold drier level achieved for the new HX adsorbent media. The actual downtime will be dependent on purifier downstream element conditions like temperature, tubing length and gas flow path. However, for a contamination controlled distribution system relative tool downtime will reduce. This in turn will allow maximum use of purifier lifetime towards process cycle steps like precursor deposition or tool cleaning.
Figure 2. A two-fold drier level was achieved with the new HX adsorbent media.
Purifier moisture lifetime, capacity
A comparison study of H and HX adsorbent media performance for < 1 ppbv level moisture removal under a high moisture challenge, 12 ppmv, at a high gas flow rate was conducted using a standard Entegris 70KF sized purifier body. High moisture challenge and gas flow rates were chosen to both allow accelerated testing and match bed volume rates of larger sized purifiers. Capacity tests were performed per SEMI- F67-1101 document . Outlet levels of < 100 pptv were observed. As the inlet moisture concentration front move across the bed the purifier lifetime reduces and leads to a "breakthrough" on reaching the purifier outlet.
The superior performance for media HX is shown by way of a normalized frequency plot of purifier outlet moisture concentration collected over the lifetime of the purifier (in this example 5 days), prior to breakthrough (Fig. 3). A peak value of 30 pptv outlet purity level is recorded with a full width half maximum (FWHM) scatter of 25 pptv. No attempt has been made to signal average the data and the distribution is a convolution of the instrument and purifier response. Deviations in the 60-300 pptv range are of the order of 5% and attributed to instrument fluctuation.
Figure 3. A normalized frequency plot of purifier outlet moisture concentration over the lifetime of the purifier.
Media H in comparison has an average outlet removal level of < 100 pptv with a FWHM of 50 pptv. HX provides greater purity stability; a key metric identified in the ITRS 2010 roadmap update for yield enhancement challenges at 16nm device size fabrication.
A high performing media for inert and hydrogen gas purification under high contaminant moisture challenge and flow is developed using state-of-art analytical instrumentation to probe sub-100 pptv levels. Comprehensive data on a wide range of contaminant removal metrics like purifier lifetime, outlet purity, removal efficiency, drydown response and outlet purity stability is presented to emphasize the interplay between metrology, standardized test methods and material improvement for gas purification advantage. Gains made in HX gas purifier performance should facilitate yield improvements in semiconductor manufacturing processes. Evaluations under hydrogen gas are underway.
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Abneesh Srivastava received his PhD degree in physical chemistry from Columbia University, New York, NY. He is Research Scientist, Gas Microcontamination Control Division at Entegris, Inc. 10070 Willow Creek Road, San Diego, CA 92131; 857.636.8010; firstname.lastname@example.org. Thomas Gaffney received his PhD degree in inorganic chemistry from Northwestern University, Evanston, IL. He is R&D Director, Gas Microcontamination Control Division at Entegris, Inc. 10070 Willow Creek Road, San Diego, CA
Solid State Technology, Volume 55, Issue 2, March 2012