Vacuum Bakeout Research Articles

Comparison between purged and vacuum bake-out efficiency

Purged bake-outs can be an appealing alternative to vacuum bake-outs to mitigate molecular outgassing of space hardware. However, their suitability is still debated since there is a lack of knowledge regarding their efficiency. This publication presents a method to assess the efficiency of purged bake-outs, comparing them to vacuum bake-outs, and provides first results of a parametric study on a few materials, with particular emphasis in the effects that pressure, nitrogen flow, and gas inlet temperature have on the efficiency of the process. The study was done without a specific mission in mind, pre-determined temperature and duration profiles were applied. It is however stressed that the actual bake-out conditions will have to be related to the particular mission of interest.

Evaluation of advanced cleaning processes for sensitive surfaces in optical instrumentation

Contamination has become a major issue in all high-tech industries, with increasingly stringent cleanliness requirements. It is the case for sensitive parts of space flight hardware, such as optics or other embedded equipment for which surface cleanliness is a critical topic, since even a thin layer of contaminant can adversely affect performance. Sometimes, cleaning may be needed to reach a specified cleanliness surface level at different integration phases of instrument or satellite, or as a corrective action in case of anomaly. Thus it is recommended to consider cleaning techniques without direct contact with the sensitive or fragile surface to prevent any damage after treatment. Five processes or techniques among those described for instance in the ECSS standard dedicated to ultracleaning of flight hardware (laser, vacuum bakeout, carbon dioxide (CO2) snow, supercritical CO2 and plasma) have been chosen for a test campaign to assess their efficacy on representative samples of sensitive optical parts (e.g.: glasses, mirrors, treatments, coatings, etc.). The study has been focused on the removal of molecular residues from substrates intentionally contaminated by a mixture of typical chemical contaminants. After a description of the selected cleaning techniques and their pros and cons, this paper will present the methodology adopted for the process validation using analytical chemistry and the comparative results of the performance of each cleaning technique. The impact on materials and the ease of implementation have been evaluated as well. In fine, the objective of this study is to provide some guidance to choose one or several non-contact processes suitable for cleaning sensitive space hardware, in order to guarantee contamination-free surfaces before flight and limit any risk of failure.

Factors influencing the gas bubble evolution and the cristobalite formation in quartz glass Cz crucibles for Czochralski growth of silicon crystals

Long-term Czochralski (Cz) silicon crystallization processes like multi-pulling demand high qualities and performances of fused quartz crucibles. Crucial issues are the gas bubble evolution and cristobalite (Crs) formation within the bulk and on the inner crucible surfaces. Here, crucibles by four suppliers were investigated using vacuum bake-out tests (VBT), for which crucible samples are heat-treated under vacuum at 1450 °C for different holding times with and without direct Si contact.An improved data analysis showed that the bubbles in all crucibles grow with process time; however, the expansion is more pronounced in the bulk material than in the critical surface-near region. Further, we observed that the final bubble size is higher in natural than in synthetic SiO2 material.Crs formation could be observed at all crucibles' surfaces, distinguishable into brownish Crs at sites with Si contact and white Crs without Si contact. The resulting Crs layer thickness and the surface covering increased with longer holding times; however, too thick layers could cause chipping. Further, the observed Crs growth rate is different for all crucible types. Finally, the crucibles were evaluated for their highest application potential for long-term Cz experiments.

Investigation of gas bubble growth in fused silica crucibles for silicon Czochralski crystal growth

Abstract Gas bubbles in crucibles for Czochralski (Cz) silicon growth are both necessary and detrimental: In the outer, bubble-containing (BC) layer of the crucible, they are required for mechanical stability, while in the inner, bubble-free (BF) layer, bubbles can cause the release of particles from the crucible into the melt which may disrupt the single-crystalline growth. In this work, a vacuum bake-out test (VBT) procedure was set up for unused crucible parts and a microscopic characterization routine was developed to systematically investigate bubble formation and growth. Longer process time, higher temperature, and lower atmospheric pressure lead to an increased bubble growth in both, the BC and BF layer. During the VBT, no new bubbles form in the BF layer, while existing bubbles grow. The comparison to experimental data from crucibles used in an industrial Cz process indicates that VBTs can simulate this process. This allows the prediction of the gas-bubble formation in Cz crucibles using a cost-effective and less time-consuming analyzation method.

Outgassing Environment of Spacecraft: An Overview

With the advancements of materials science and technology, organic materials found significant applications in aerospace industry. But under the orbital thermal and vacuum environment, organic materials can outgas and the outgassed products and induced environment often degraded performances of spacecraft or its subsystem. In this article, the typical impacts of outgassing environment on spacecraft are explained briefly. Then the outgassing theory, test method, identification of compounds and control measures were reviewed in detail. The need for transient and long-term outgassing model is discussed. The standard test methods of E595 and E1559 are compared. The promising new techniques such as infrared spectrometry and/or mass-spectrometry with thermogravimetric analysis is proposed for the identification of outgassed compounds. The vacuum bakeout and molecular absorber for outgassing control are reviewed and for the last more research work is needed.

Outgassing behavior of different high-temperature resistant polymers

In vacuum applications, polymers often need to be used in spite of their relatively high outgassing rates, e.g., as sealing or spacer material. For some polymers—like Viton®, which is used for O-rings—data on outgassing rates can be found in literature, while for others, in particular, the most relevant ones that can be used at higher temperatures, only few results are available. Therefore, different high-temperature resistant polymers were measured at the Outgassing Measurement Apparatus, which uses the difference method. These polymers were polyimides like Kapton®, Vespel®, and Meldin®, as well as a polyetheretherketone and a polybenzimidazole. In order to investigate the temperature behavior of the outgassing of the polymers, the measurements were performed at room temperature, 100 °C, and 200 °C. The outgassing species were determined by a mass spectrometer. In addition, the dependence of the outgassing on the material thickness was analyzed. The paper shows that the outgassing rates of the examined polymers are higher than 10−4 (Pa m3)/(s m2) after 10 h at room temperature, except for the thin Kapton samples. Thus, further treatment of the materials will be necessary depending on the application. It is shown that a vacuum bake-out at not less than 200 °C is favorable. The main outgassing species is water, as expected, but also different hydrocarbons with higher mass can be seen for the polyimides. Concerning different thicknesses, the outgassing rate of a thin sample generally decreases more steeply than the outgassing rate of the thicker sample. Nevertheless, the steepest decrease was found for the thickest Kapton sample, with a difference of nearly factor 500 between 10 and 100 h at room temperature.

In situ oxygen plasma cleaning of microswitch surfaces—comparison of Ti and graphite electrodes

Ohmic micro- and nanoswitches are of interest for a wide variety of applications including radio frequency communications and as low power complements to transistors. In these switches, it is of paramount importance to maintain surface cleanliness in order to prevent frequent failure by tribopolymer growth. To prepare surfaces, an oxygen plasma clean is expected to be beneficial compared to a high temperature vacuum bakeout because of shorter cleaning time (<5 min compared to ~24 h) and active removal of organic contaminants. We demonstrate that sputtering of the electrode material during oxygen plasma cleaning is a critical consideration for effective cleaning of switch surfaces. With Ti electrodes, a TiOx layer forms that increases electrical contact resistance. When plasma-cleaned using graphite electrodes, the resistance of Pt-coated microswitches exhibit a long lifetime with consistently low resistance (<0.5 Ω variation over 300 million cycles) if the test chamber is refilled with ultra-high purity nitrogen and if the devices are not exposed to laboratory air. Their current–voltage characteristic is also linear at the millivolt level. This is important for nanoswitches which will be operated in that range.

우주환경하의 위성부품용 압전진동자 활용에 관한 연구

위성체가 작동하는 우주환경인 초고진공상태에서는 각 부품에서 발생 할 수 있는 기체방출로 인해 위성체가 오염되어 위성체의 성능이 저하될 수 있으며, 특히 광학렌즈를 오염시킴으로써 위성체 본연의 임무수행 실패라는 결과를 초래할 수도 있다. 최근 항공우주분야에서 활용 범위가 넓어지고 있는 PZT-5 계열 압전진동자의 위성부품 활용성 연구를 위하여 먼저 Collected Volatile Condensable Material 및 Total Mass Loss을 측정하여 규정된 0.1% 및 1.0% 이하의 값을 얻었고, 한국항공우주연구원에 설치된 베이크아웃(bake-out) 챔버를 이용하여 고온 및 고진공상태에서 500 <TEX>$ng/cm^2/hr$</TEX> 이하의 낮은 Thermoelectric Quartz Crystal Microbalance 값을 얻어 위성체 부품으로의 적합성을 재확인하였다. 압전진동자에 대한 고진공 환경 전후의 압전 특성을 비교 분석한 결과 진공환경에 의한 전기-기계적 특성은 1% 미만으로 큰 변화가 없음을 확인하였다. In the high vacuum condition of the space, outgassing from any assembly of satellite can contaminate satellites, especially second surface mirror and optical lens, it cause satellite to fail in own missions. Therefore, all unit shall be check for compatibility with vacuum using CVCM (Collected Volatile Condensable Material) and TML (Total Mass Loss) in advance. CVCM and TML of the PZT-5 piezoelectric ceramic vibrator has less than 0.1% and 1.0% respectively. Also, it has less than 500 <TEX>$ng/cm^2/hr$</TEX> of Thermoelectric Quartz Crystal Microbalance for vacuum bake-out test using high temperature(more than <TEX>$80^{\circ}C$</TEX>) and high vacuum (less than <TEX>$5.0{\times}10^{-3}$</TEX> Pa). Thus, piezoelectric ceramic vibrator may be employed in the vacuum environments. Finally, it can be confirmed that the characteristics change of the piezoelectric ceramic vibrator is less than 1% under vacuum environments.

Design, fabrication, performance and reliability of Pt- and RuO2-coated microrelays tested in ultra-high purity gas environments

We have fabricated, tested and analyzed the reliability of Pt- and RuO2-coated ohmic microrelays in ultra-high purity gas environments. RuO2-coated relays could survive 3 × 108 contact cycles without electrical degradation, while Pt-coated devices degraded after 105 cycles. Thermally actuated microrelays were fabricated using a process that employed a polysilicon surface-micromachined substructure. After releasing the devices, just a few blanket metal depositions were required to create the different coatings. This method allowed direct comparisons between different coating materials, and was enabled by a self-aligned shadow mask that provides electrical isolation between different traces. Testing was performed in a clean environment achieved through in situ ultra-high vacuum bakeouts, chamber cooling to <5 × 10−9 Torr and chamber refill with ultra-high purity gases. The RuO2 coatings were formed by two avenues—reactive sputtering and thermal oxidation. No significant difference in contact resistance or reliability was detected for these two deposition methods. For all coatings, post-test analysis by scanning electron microscopy and Auger electron spectroscopy indicated no difference in carbon concentration on real contact versus non-contacting areas, implying that carbon did not play a role in limiting the switches’ reliability. The Pt-coated switch reliability limit was attributed to surface wear rather than to the growth of a contaminating film. For the RuO2 switches, trace resistance was reduced by ten times using an Al underlayer, so that the total device resistance was compatible with commercial device requirements. Because RuO2 is expected to be resistant to hydrocarbon contamination, this work shows that the RuO2 coating provides a promising path toward achieving ultra-high reliability ohmic microswitches.

Measurement of Molecular Contamination for Satellites Using a Quartz Crystal Microbalance (QCM)

Quartz Crystal Microbalances (QCMs) have been used to measure the molecular contamination for satellites in vacuum bake-out tests at the Korea Aerospace Research Institute (KARI). Through measurement using QCMs, the mass and the mass deposition rate of molecular contamination could be traced in real-time. Vacuum bake-out tests were performed for the satellite KAISTAT-4 Flight Model and its components under various temperature conditions. During these tests, QCM measurement data was obtained and materials from the cold plate were analyzed using a Gas Chromatograph-Mass Spectrometer (GC-MS). The characteristics of the QCM sensor were also observed.

Development of CVD diamond r.f. windows for ECRH

The recent development of CVD diamond windows for high power gyrotrons is summarized. Highlights of this work include the manufacture of a window 100 mm in diameter and 1.6 mm thick having a uniform loss tangent of <1×10 −5 at 145 GHz, and a window 120 mm in diameter and 2.25 mm thick with losses below 3×10 −5 at 145 GHz. Moreover, the dielectric loss and permittivity have been found to be relatively stable over a wide temperature range from 100 to 700 K compared to other materials such as sapphire or gold doped silicon. More recently, development work has concentrated on achieving a robust and cost effective manufacturing process able to produce windows matching the requirements of 1 MW CW gyrotrons for ECRH applications. Typical dielectric properties for this new grade of material are presented using a 110 GHz gyrotron window 114 mm in diameter and 1.715 mm thick as an example. Data show a loss tangent in the region of 2×10 −5 at 145 GHz across the 80 mm clear aperture region of the disc. A numerical model is used to calculate the temperature and stress profile of the window when subjected to high power beam conditions. It is shown that these dielectric properties, in conjunction with a thermal conductivity in the region of 2000 W m −1K −1 at room temperature and a fracture strength in excess of 250 MPa, make this grade of diamond suitable for use as an output window on 1 MW CW gyrotrons. A new method of mounting CVD diamond windows to metal flanges has been developed to allow vacuum bakeout to be performed at 550°C compared to the previous limit of 450°C. This increase in bakeout temperature promises dramatic reductions in the time taken to achieve optimum vacuum conditions in the gyrotron. The new mounting utilizes a novel graded seal arrangement to achieve low thermal mismatch stresses and good dimensional stability during thermal cycling. A 75 mm diameter window mounted using this method has been shown to survive five thermal cycles up to 550°C for 1 h without degradation of its sealing performance.

A study of thermal treatment procedures to reduce hydrogen outgassing rate in thin wall stainless steel cells

Stainless steel is the most frequently used material for manufacturing of vacuum chambers and components. Recombination was shown to be a rate-limiting mechanism in kinetics of hydrogen outgassing at low concentration. Unfortunately, published data of recombination rate coefficient, K L , are poorly correlated with surface composition. This makes numerical modelling uncertain and it is difficult to compare the efficiency of different pre-treating methods; vacuum annealing, air oxidation, etc. Thin walled stainless steel test cells (wall thickness 0.15 mm, V=0.16 l, A=460 cm 2) were processed in situ by moderate vacuum bake-out. Pressure measurements in the cells by a spinning rotor gauge (SRG) started at least 24 h after they were sealed-off. Gas accumulation time was typically a few days. The hydrogen outgassing rate was determined from the slope of the total pressure rise. The range of q out below 2×10 -14 mbar l s -1 cm -2 was routinely obtained at room temperature. Additionally, two different pre-treatment methods were applied, hydrogen annealing and oxidation in air, but no evident influence to final outgassing rate was observed. Surface composition was analysed by AES before and after pumping in all cases. Correlation of these data and the resulting outgassing rates are discussed.

Effects of energy deposition by nuclear scattering in silicon p-i-n diode detectors

The changes in thick (∼300 μm) and thin (15 μm) Si p-i-n diode detectors that take place as a result of alpha particle and 63Cu 5+ ion irradiation have been studied by Deep Level Transient Spectroscopy (DLTS) and by monitoring the reverse-bias leakage currents. A linear increase in reverse-bias current with α-particle dose was observed that could be attributed to the formation of vacancy-associated defects (divacancy, A- and E-centres). Damage within the active layer of the device, characterised by the increase in leakage current per unit active volume, together with literature pion, proton and heavy ion data, exhibited a linear dependence on the energy deposited in nuclear processes over many orders of magnitude. Annealing at temperatures of 150°C for 4 h (vacuum bakeout) resulted in a reduction in leakage current and the size of the DLTS peaks became smaller. This temperature is much lower than expected for removal of divacancies, suggesting that recovery of the reverse bias current is mediated by other defect centres.

Contamination Effects on the Geostationary Operational Environmental Satellite Instrument Thermal Control System

The instrument thermal control system of the Geostationary Operational Environmental Satellite may have been affected by specie c contamination problems that arose because of the unique conditions and requirements of the spacecraft mission. This paper addresses some specie c contamination effects from the coatings used in the instrument cavities. Contamination control actions that were implemented during ground processing to ensure limited impact on the on-orbit temperature control are described. The selection of thermal coatings is an integral part of the overall spacecraft design. Molecular contamination accretion on thermal coatings may alter the design properties of the surface coatings. In an effort to quantify the molecular contamination effects from material outgassing, an assessment was conducted to address the concerns inside the instrument cavities. The study results prompted an extensive prelaunch vacuum bakeout effort and an on-orbit solar radiation avoidance exercise. In addition, the thermal performance of the instrument radiant coolers could have been detrimentally affected by scattered solar radiation from particulate contamination. To mitigate the impact of the particulate contamination the radiant cooler surfaces were cleaned to an established criteria prior to launch.

Analysis of coatings which inhibit epoxy bleeding in electronic packaging

The bleeding of epoxy resin around surfaces, undergoing bonding in electronic packaging assembly, has for long caused sporadic yield-loss. This loss is more severe in advanced and dense packages. Previously it has been supposed that the loss results from surface contaminants which are reduced by vacuum bakeout. In fact, that procedure generates coatings of hydrocarbons as we show by surface analysis. The coatings very strongly affect the wettabilities and measured surface energies of substrates. In particular, the comparatively low surface energy of hydrocarbon films on gold surfaces shows how the surfaces can be engineered, with coatings of “appropriate cleanliness”, to systematically inhibit epoxy bleeding.

Contamination effects on the Geostationary Operational Environmental Satellite Instrument thermal control system

The instrument thermal control system of the Geostationary Operational Environmental Satellite may have been affected by specific contamination problems that arose because of the unique conditions and requirements of the spacecraft mission. This paper addresses some specific contamination effects from the coatings used in the instrument cavities. Contamination control actions that were implemented during ground processing to ensure limited impact on the on-orbit temperature control are described. The selection of thermal coatings is an integral part of the overall spacecraft design. Molecular contamination accretion on thermal coatings may alter the design properties of the surface coatings. In an effort to quantify the molecular contamination effects from material outgassing, an assessment was conducted to address the concerns inside the instrument cavities. The study results prompted an extensive prelaunch vacuum bakeout effort and an on-orbit solar radiation avoidance exercise. In addition, the thermal performance of the instrument radiant coolers could have been detrimentally affected by scattered solar radiation from particulate contamination. To mitigate the impact of the particulate contamination the radiant cooler surfaces were cleaned to an established criteria prior to launch.

Ion desorption stability in superconducting high energy physics proton colliders

In this article we extend our previous analysis of a cold beam tube vacuum in a superconducting proton collider to include ion desorption in addition to thermal desorption and synchrotron radiation induced photodesorption. The ion desorption terms introduce the possibility of vacuum instability. This is similar to the classical room temperature case but is now modified by the inclusion of ion desorption coefficients for cryosorbed (physisorbed) molecules which can greatly exceed the coefficients for tightly bound molecules. The sojourn time concept for physisorbed H2 is generalized to include photodesorption and ion desorption as well as the usually considered thermal desorption. The ion desorption rate is density dependent and divergent so at the onset of instability the sojourn time goes to zero. Experimental data are used to evaluate the H2 sojourn time for the conditions of the Large Hadron Collider (LHC) and the situation is found to be stable. The sojourn time is dominated by photodesorption for surface density s(H2) less than a monolayer and by thermal desorption for s(H2) greater than a monolayer. For a few percent of a monolayer, characteristic of a beam screen, the photodesorption rate exceeds the ion desorption rate by more than two orders of magnitude. The photodesorption rate corresponds to a sojourn time of approximately 100 s. The article then turns to the evaluation of stability margins and the inclusion of gases heavier than H2 (CO, CO2, and CH4), where ion desorption introduces coupling between molecular species. Stability conditions are worked out for a simple cold beam tube, a cold beam tube pumped from the ends, and a cold beam tube with a coaxial perforated beam screen. In each case a simple inequality for stability of a single component is replaced by a determinant that must be greater than zero for a gas mixture. A connection with the general theory of feedback stability is made and it is shown that the gains of the diagonal uncoupled feedback loops are first order in the ion desorption coefficients whereas the gains of the off-diagonal coupled feedback loops are second order and higher. For this reason it turns out that in practical cases stability is dominated by the uncoupled diagonal elements and the inverse of the largest first-order closed loop gain is a useful estimate of the margin of stability. In contrast to the case of a simple cold beam tube, the stability condition for a beam screen does not contain the desorption coefficient for physisorbed molecules, even when the screen temperature is low enough that there is a finite surface density of them on the screen surface. Consequently there does not appear to be any particular advantage to operating the beam screen at a high enough temperature to avoid physisorption. Numerical estimates of ion desorption stability are given for a number of cases relevant to the LHC and all of the ones likely to be encountered were found to be stable. The most important case, a 1% transparency beam screen at ∼4.2 K, was found to have a stability safety margin of approximately 17 determined by ion desorption of CO. Ion desorption of H2 is about a factor of 75 less stringent than CO. For these estimates the beam tube surface was assumed to be chemically cleaned but otherwise untreated, for example, by a vacuum bakeout or by glow discharge cleaning.

Water vapor in vacuum systems

A major concern for vacuum processing equipment designed to operate in the high and ultrahigh vacuum range, is the outgassing from the walls of the vacuum chamber. Whenever a vacuum system is opened to air, all the internal surfaces will be covered with layer upon layer of water molecules. The thickness of the layer will depend on the relative humidity of the air, the exposure history, etc. After a few hours of pumping down the system, water desorption from the walls will slow down and pressure will remain steady until the last layer of water is depleted. The economical way to reduce the desorption rate and remove large amounts of water vapor, is to use methods for conditioning the construction materials and transfer energy to the adsorbed gases to break their bonds. Traditional vacuum bakeout cannot be used with some applications such as accelerators, or fusion devices. Several other methods such as flushing with cold or hot neutral gas, glow discharge, or either electron, ion, or ultraviolet radiations have been successfully used. The problem of water vapor will be discussed in detail.

Bake stability of long-wavelength infrared HgCdTe photodiodes

The bake stability was examined for HgCdTe wafers and photodiodes with CdTe surface passivation deposited by thermal evaporation. Electrical and electrooptical measurements were performed on various long-wavelength infrared HgCdTe photodiodes prior to and after a ten-day vacuum bakeout at 80°C, similar to conditions used for preparation of tactical dewar assemblies. It was found that the bakeout process generated additional defects at the CdTe/ HgCdTe interface and degraded photodiode parameters such as zero bias impedance, dark current, and photocurrent. Annealing at 220°C under a Hg vapor pressure following the CdTe deposition suppressed the interface defect generation process during bakeout and stabilized HgCdTe photodiode performance.

Evaluation of different cleaning techniques for oxygen-free high-conductivity copper

Oxygen-free high-conductivity copper is a candidate material for future electron/positron storage rings with beam currents in the range of ampere or more. The advantages being high electrical conductivity and good x-ray absorption. However, there are problems of fabrication and lack of extensive data on easy cleaning techniques for complicated shapes. We have over the last year gained considerable experience and understanding of the same. Techniques varying from conventional chemical cleaning, high-pressure hot-water jet cleaning, and vacuum bakeout, to novel methods such as CO2 snow cleaning, and wet bead blasting have been tried. The samples were evaluated for surface cleanliness by x-ray photoelectron spectroscopy. Data along with some analysis and interpretation are being presented.