Volume 9, Issue 6, November 1991
Index of content:
Recommended practices for the calibration and use of capacitance diaphragm gages as transfer standards9(1991); http://dx.doi.org/10.1116/1.577209View Description Hide Description
Capacitance diaphragm gages which were previously known as capacitance manometers, have become a ubiquitous part of systems used in the vacuum community because of their ease of use, compatibility with most gases, and potential accuracy and resolution. They are used not only as pressure measuring devices, but also as transfer standards against which other pressure instruments are calibrated. There are a variety of precautions that must be considered when using these instruments. The American Vacuum Society is therefore presenting this Recommended Practice as a guide to their proper calibration and use as transfer standards. The document covers suggested plumbing configurations, laboratory practices, calibration methods, and reports.
High mass resolution surface imaging with a time‐of‐flight secondary ion mass spectroscopy scanning microprobe9(1991); http://dx.doi.org/10.1116/1.577145View Description Hide Description
This article describes first applications of a time‐of‐flight secondary ion mass spectroscopy (TOF‐SIMS) scanning microprobe, based on a high mass resolution TOF‐SIMS instrument, combined with two pulsed primary ion sources: (a) 10 keV Ga liquid metalion source (LMIS), probe size: 0.5–1 μm; (b) 10 keV electron impact (EI) ion source (Ar+,Xe+,O+ 2), probe size 4–10 μm. The detection limits for elemental and molecular surface species as a function of probe size are discussed. At a lateral resolution of 1 μm secondary ion images with about 1000 counts/pixel can be acquired in about 30 min. The high useful yields achieved by TOF‐SIMS allows the analysis of submonolayers of inorganic as well as organic materials at high lateral resolution. Currently up to 24 secondary ion images for different masses and a complete mass spectrum can be acquired simultaneously. The performance of the instrument is demonstrated by multielemental and molecular imaging of inorganic and organic patterns on Si wafers. Secondary ion images of a polymer with an average molecular weight of 1400 amu prepared on a silver substrate are presented. As an example for the analysis of insulating materials secondary ion images of an electrically isolated surface mounted device solder pad on an electronic board are shown.
Kinetics of thermal decomposition of triethylgallium, trimethylgallium, and trimethylindium adsorbed on GaAs(100)9(1991); http://dx.doi.org/10.1116/1.577146View Description Hide Description
We report studies of the kinetics of thermal decomposition of triethylgallium (TEGa), trimethylgallium (TMGa), and trimethylindium (TMIn) adsorbed on GaAs(100) in ultrahigh vacuum. The adsorbed layers were prepared by dosing GaAs(100) at room temperature, to either saturated coverage or coverages below saturation. The relative coverage of carbon was monitored by x‐ray photoelectron spectroscopy(XPS) as the substrate temperature was slowly increased (0.6–3.2 °C/min). Products were detected at faster heating rates (0.7–6 °C/s) with a differentially pumped quadrupolemass spectrometer. The substrate temperature was measured by infrared laser interferometric thermometry. The kinetic analysis also makes use of XPS and mass spectrometric data on laser‐induced, rapid thermal decomposition (heating rates of ∼1011 °C/s ). TEGa dissociatively chemisorbs on GaAs(100) at room temperature. Heating the substrate from room temperature to ∼500 °C results in desorption of a Ga–alkyl at low temperature, ascribed mostly to diethylgallium (DEGa) and possibly some TEGa.
At higher temperature, C2H4 and C2H5desorb in parallel after most of the Ga–alkyl has desorbed. The hydrocarbon desorption is described well by simple first order kinetics with an activation energy,E act=32±4 kcal/mol, and a pre‐exponential A factor of 2.5×1010±1.5 s−1. Ga–alkyl desorption is more complicated; the Arrhenius parameters for assumed first order desorption exhibit strong coverage dependences. A fit to all the data was obtained for A=5×108 s−1 and E act=18 kcal/mol at saturated coverage, with a large decrease in E act (or increase in A) with decreasing coverage. TMGa decomposes to yield a Ga–alkyl desorption product (either dimethylgallium, TMGa, or a mixture of the two) at low temperature, and CH3 at higher temperature. CH3desorption has a first order activation energy of 43±2 kcal/mol for an assumed A factor of 1×1013 s−1. For the Ga–alkyl, A=108 s−1 and E act=19 kcal/mol, with a coverage dependence similar to DEGa desorption from TEGa decomposition. TMIn undergoes a methyl exchange reaction on GaAs(100). Upon heating above room temperature, a Ga–alkyl desorbs first, followed by desorption of CH3 at higher temperature. The Ga–alkyl has with the same cracking pattern as observed for TMGa decomposition. No In–alkyls desorb, and In desorption does not occur until all carbon‐containing species desorb. CH3 starts to desorb at lower temperature than for TMGa decomposition. Assuming an A factor of 1×1013 s−1, CH3desorption over the observed wide temperature range indicates a range of activation energies from 33–43 kcal/mol. Ga–alkyl desorption is similar to that observed during TMGa decomposition. At saturated coverage, A=108 and E act=17 kcal/mol. However, the coverage dependence is not as strong as for TMGa, so that Ga–alkyl desorption peaks at lower temperature for TMIn. Decomposition mechanisms for these group‐III metal alkyls are discussed, along with implications for growth of III–V compound semiconductor films from these precursors by chemical vapor deposition and molecular beam techniques.
Products of pulsed laser induced thermal decomposition of triethylgallium and trimethylgallium adsorbed on GaAs(100)9(1991); http://dx.doi.org/10.1116/1.577147View Description Hide Description
Products of pulsed laser‐induced decomposition of triethylgallium (TEGa) and trimethylgallium (TMGa) chemisorbed on GaAs(100) were determined and compared to those observed in thermal desorption. The desorbing products, C2H5, C2H4, and Ga(C2H5) x (x=2 and/or 3) for TEGa decomposition, and CH3 and Ga(CH3) x (x=2 and/or 3) for TMGa, are the same as those produced in thermal desorption. However, rapid substrate heating with the pulsed excimer laser causes large changes in the relative yields. With laser induced heating, Ga‐alkyl desorption is almost completely suppressed in favor of further decomposition to yield hydrocarbon products and leave Ga on the surface, while in thermal desorption the Ga‐alkyl is a major product. Furthermore, for laser‐induced thermal decomposition of TEGa, C2H5 production is enhanced with respect to C2H4. These effects are ascribed to decomposition of chemisorbed GaR2(ads) (R=CH3, or C2H5), leading to formation of adsorbed GaR(ads) and desorption of R(g). This decomposition reaction is entropically favored over desorption of the Ga‐alkyl, but has a higher activation energy. Consequently, decomposition is favored at the rapid heating rates, while desorption of GaR x dominates at slow heating rates. Competition between these desorption and decomposition reactions is likely important in the overall conversion of TEGa and TMGa to expitaxially grown, Ga‐containing III–V compound semiconductor films. Arrhenius parameters are presented for the GaR2decomposition reactions. These data should improve the accuracy of kinetic models for epitaxial film growth by chemical vapor deposition and molecular beam techniques.
9(1991); http://dx.doi.org/10.1116/1.577148View Description Hide Description
Trimethylgallium (TMGa) chemisorption on the gallium‐rich ‘‘(4×6)’’ and (1×6) GaAs(100) surfaces has been studied with temperature programmed desorption(TPD) and static secondary ion mass spectroscopy (SSIMS). Low coverages of TMGa irreversibly dissociate on GaAs(100) and methyl radicals are the main gas‐phase product observed around 700 K during TPD. At higher TMGa coverages some recombinative desorption of molecular TMGa occurs from 250–650 K. On the Ga‐rich surface reconstructions the desorbing methyl radicals are kinetically indistinguishable, strongly suggesting that they evolve from a single type of binding site. We believe this binding site corresponds to surface MMGa, which is formed by stepwise methyl group transfer from chemisorbed TMGa to neighboring surface gallium atoms. The presence of MMGa is supported by SSIMS results at ∼650 K. Temperature‐programmed SSIMS was used to follow the dissociation of adsorbed TMGa from 120 to 700 K.
X‐ray photoelectron spectroscopy studies of contamination and cleaning of surfaces exposed to a fluorocarbon plasma9(1991); http://dx.doi.org/10.1116/1.577149View Description Hide Description
A reactive ion etching (RIE) with a mixture of fluorocarbon gases is used to etch selectively SiO2patterns on Si wafer. X‐ray photoelectron spectroscopy(XPS) measurements were carried out to determine the walls contamination of the reactor with different cathode coatings like Al, Si, and photoresist. The contamination of different wafers (SiO2, Si with and without resist) following RIE processes was also studied in order to understand the mechanism of the selective etching. Then different cleaning processes, in particularly NF3 and O2plasmas followed or not by a hydrophilic treatment, were also qualified by XPS. For every sample, the Al(2p) or Si(2p), C(1s), O(1s), and F(1s) photoelectron lines were monitored and concentrations determined as usual by integration of these lines. F(1s) and C(1s) were decomposed into several components corresponding to different chemical bonds. The results show that the contamination of the reactor walls by fluoropolymers is due to the presence of Si or of the photoresist on the cathode; there is no polymerformation with an Al cathode coating. The polymer contamination on the reactor walls is cleaned off by an O2plasma and the residual fluorine forms a passivative layer with the Al of the reactor walls. During the plasma etching of a SiO2 film, there is formation of C–Si, C–C, O–Si–F, F–Si bonds but it is only when there is no oxygen left that CF x bonds are formed. These polymer layers stop the etching process on silicon. They are cleaned by O2 or NF3plasma, but the latter removes the polymers without increasing the oxygen, fluorine or nitrogen contamination and appears to be most appropriate for improving the surface quality of silicon after fluorocarbon plasma etching.
9(1991); http://dx.doi.org/10.1116/1.577150View Description Hide Description
The adhesion between 30 nm Ni films and quartz substrates was examined as a function of the interfacial chemistry and mixing induced by low temperature (<100 °C) 40Ar+ and 28Si+ ion mixing. The Ni/quartz specimens were implanted with either 65 keV 40Ar+ or 55 keV 28Si+ to doses between 1 and 10×1016 atom/cm2. Both types of implants were observed to induce extensive interfacial grading. X‐ray photoelectron spectroscopyanalyses indicated that Ni–O–Si types of interfacial complexes were present in the as‐deposited specimens, but that these complexes were eliminated by the 40Ar+ ion mixing. In contrast, high dose 28Si+ ion mixing was shown to induce the formation of new interfacial complexes (Ni–Si–O and new Ni–O–Si complexes). Adhesion measurements, performed using a scratch tester, indicated that the adhesion of the as‐deposited specimens was good (20 N), but that this adhesion got progressively worse (to less than 1 N) with 40Ar+ implantation. In the case of 28Si+ ion mixing,adhesion was reduced (to 5 N) in the low dose specimens, but was substantially increased (to 45 N) in the high dose specimens. Overall, Ni/quartz adhesion was shown to correlate with the concentration of interfacial complexes that could chemically attach the Ni films and quartz substrates.
9(1991); http://dx.doi.org/10.1116/1.577151View Description Hide Description
The reaction of dimethylzinc on Si(100)‐2×1 was studied using temperature programmed desorption(TPD) and high resolution electron energy loss spectroscopy (HREELS). Dimethylzinc dissociated on this surface at temperatures less than 400 K producing zincmetal and surface methyl groups. During TPD experiments the zincmetal desorbed completely by 600 K, leaving only methyl groups on the surface at higher temperatures. The predominate decomposition pathway for the surface methyl species was complete dehydrogenation producing molecular hydrogen and depositing carbon on the surface. HREELS results indicated that methyl group dehydrogenation occurred at temperatures greater than 700 K. For high initial coverages of dimethylzinc, in addition to complete dehydrogenation products, methyl radicals and acetylene were also detected. The hydrogen desorption temperature was found to increase as a function of the amount of carbon deposited on the surface.
Laser induced photodesorption of SiCl from Si(100) monitored by time of flight and time resolved reflectivity9(1991); http://dx.doi.org/10.1116/1.577152View Description Hide Description
The excimer laser induced desorption rate of SiCl molecules from a chlorinated siliconsurface is measured in real time together with the transient change of the surfacereflectivity at 632.8 nm. It is demonstrated that, under conditions where gas‐phase chlorine is not photoexcited and does not collide with the surface during the laser pulse, laser induced desorption occurs only when the surface melts, at the laser wavelengths 308 and 248 nm.
9(1991); http://dx.doi.org/10.1116/1.577153View Description Hide Description
The effects of ion irradiation on polyimide surfaces have been studied using x‐ray photoemission techniques. Ion bombardment with energies in the range 0.5–2.0 keV and doses between 8×1013 and 1×1015 ions/cm2 were carried out i n s i t u in the x‐ray photoelectron spectrometer and the chemistry of the modified surface was monitored using core level spectral changes. At low doses and energies, carbonyl groups were preferentially sputtered keeping the rest of the monomer intact. Loss of nitrogen was insignificant compared to losses of carbon and oxygen. At higher energies and doses, the polymer undergoes extensive bond scission, restructuring of various functional groups and species, together with radical and anion formation. High resolution spectra indicated a binding energy scale shift to a lower value, which increased with ion energy and dose, and which was related to the creation of a surface negative charge. The effects of exposure to moisture in the ambient on the surface charge, on the surface structure, and on the surface chemistry was studied.
Surface chemistry and thickness‐dependent optical properties of films of oxidized Ni/Cr on polyimide9(1991); http://dx.doi.org/10.1116/1.577154View Description Hide Description
Films were sputter deposited from a Ni/Cr target (4:1 weight ratio) in an oxygen plasma onto polyimide substrates. The films are black in color, and show excellent adhesion and flexibility. Comparison of x‐ray photoelectron spectra of 1.7 μm‐ and 3.5 μm‐thick coatings of oxidized Ni/Cr with a control coating of oxidizedNi (i.e., presumably NiO) suggests that the presence of the Cr correlates with the formation of at least one nickel species other than NiO. As the oxidized Ni/Cr film thickness varies from 0.13 to 3.5 μm, the absorptance and emittance vary from 0.78 to 0.89, and from 0.27 to 0.62, respectively. Using a model proposed by Thakur, it is calculated that for the thicker films the emitted radiation has a wavelength of approximately 7 or 8 μm.
9(1991); http://dx.doi.org/10.1116/1.577155View Description Hide Description
High‐resolution x‐ray photoelectron spectroscopy has been applied to study the formation of the merocyanine/aluminum interface. It is concluded that merocyanine exists in zwitterionic form and aluminum bonds with merocyanine at −O–C– and C=O in carbonyl group, forming the complex compound of C–O–Al. The attenuation curves of C,N,O,S exhibit the cluster growth of aluminum, which preferentially resides on heterocycles. The temperature effect on merocyanine/Al interface is also investigated in detail.
Fourier transform infrared studies of polyimide and poly(methyl‐methacrylate) surfaces during downstream microwave plasma etching9(1991); http://dx.doi.org/10.1116/1.577156View Description Hide Description
The change in chemical structure of the top surface layer of thin poly(methylmethacrylate) (PMMA) and polyimide (PI) films (200–1500 Å) during downstream microwave NF3/CF4/O2plasma etching are investigated by i n s i t uFourier transform infrared (FTIR) reflection–absorption spectroscopy. Plasma fluorination of PI by either NF3 or CF4 leads to significant surface fluorination characterized by the formation of aliphatic fluorine compounds (CF x ), acyl fluoride, benzoyl fluoride, acyl hypofluorite, and polyfluorinated benzene. The surface fluorination process is found to be controlled by diffusion in the product layer and the depth of fluorination is estimated from infrared absorbance to be approximately 500 Å. Addition of oxygen leads to a reduction in the fluoride species and the development of broad absorption bands representing oxygenated surface species. The FTIR data are supplemented by e x s i t u x‐ray photoelectron spectroscopy observations and mechanisms for the observed modifications of the polymer surfaces are proposed. The effect of humidity on the plasma‐modified polymers is studied by comparing infrared spectra collected i n s i t u after controlled exposure to moisture. PI fluorinated in NF3 show no significant changes, whereas a slow but noticeable change in the carbonyl bands is detected for NF3/Ar plasma‐fluorinated PMMA. Addition of oxygen to the microwave plasma increases the sensitivity of the modified polymer to moisture. These observations are interpreted in terms of the wettability of the polymer surface.
9(1991); http://dx.doi.org/10.1116/1.577157View Description Hide Description
The role of oxygen in affecting the adhesive bonding at the metal/polyimide (polyimide‐on‐metal) interface has been studied. Both pyromellitic dianhydride (PMDA) ‐oxydianiline (ODA) and biphenyl‐tetracarboxylic dianhydride(BPDA) ‐p‐phenylene diamine (PDA) based poly(amic acid) precursors were cast and fully imidized on metal surfaces of Cr, Cu,Ni, Co, Cu/Ni, and Cu/Co in a nitrogen atmosphere. The peel strengths at the polyimide‐on‐metal interface were measured, using a 90° peel test, immediately after curing, and then remeasured after annealing in either nitrogen (N2 −≤100 ppm O2 ) or air (N2 –21% O2 ) at 350 °C for 1 h. Very little or no change in peel strength was measured after these samples were annealed in nitrogen, while significant adhesion degradations were measured on all metals after annealing in air. The loss of polyimide (PI) adhesion to the Cu or Co surface is attributed to metal catalyzed thermal–oxidative degradation of the PI at the metal/PI interface, as characterized by polyimide thickness reduction, Fourier transform infrared and x‐ray photoelectron spectrometries. The rate of degradation of the PMDA‐ODA films on Cu or Co is several times faster than that of the BPDA‐PDA films.
The degradation products were characterized as CO2, CO, Cu carboxylate salts and a nitrile moiety entrapped in partially degraded PI films. The extent of degradation was found to increase with increasing oxygen content in the annealing ambient or with decreasing PI thickness, indicating that oxygen diffusion through the PI overcoating to the metal/PI interface plays a critical role in promoting degradation. On the Cr and Nisurfaces, such metal catalyzed degradation was not detected with our techniques, while some adhesion strength was still retained, a significant loss in adhesion strength was observed. This is probably due to oxidation of the metal underlayer causing debonding at the originally well adhered metal/PI interface. When all testing variables were held constant, the peel strength of a well adhered metal/PI joint is shown to increase with increasing peeling rate (crosshead speed). An increase of up to ∼40% in peel strength was measured when the crosshead speed was increased from a low speed of 0.05 mm/min to 10 mm/min. The locus of fracture of the as‐cured films was cohesive, i.e., the fractured surface was located within PI, while for the thermal–oxidative degraded joint the failure mode was changed to adhesive.
9(1991); http://dx.doi.org/10.1116/1.577158View Description Hide Description
Surfaces of polyethylene and polyimide films were treated in NH3 and N2plasma using ‘‘dual‐frequency’’ excitation: The samples were exposed to a microwave (2.45 GHz) glow discharge, while variable radio frequency (13.56 MHz)—induced negative direct current bias voltage values were simultaneously applied. The surface chemical structure was determined by high‐resolution x‐ray photoelectron spectroscopy. Up to 40 at. % of nitrogen was incorporated onto the sample surface by exposure to a microwave discharge in N2, while systematically lower N uptake was found in NH3plasma. Nitrogen was found to be bonded predominantly in amine (C–N) groups by NH3 ‐type plasma, and in imine groups (C=N) by N2plasma.Surface ‘‘damage’’ of polyimide, including opening of benzene rings and breaking of carbonyl groups, accompanied by the formation of increasing concentrations of chemically reactive receptor sites, has been demonstrated with increasing the energy of bombarding species.
9(1991); http://dx.doi.org/10.1116/1.577159View Description Hide Description
Factor analysis is applied to the Auger crater‐edge profile of a palladium/hydrogenated amorphous silicon layered structure. The results show, apart of a considerable improvement in interface determination, new information not obtainable by the conventional Auger depth profiling technique, which makes this procedure extremely useful for thin film depth profiling.
X‐ray photoelectron spectroscopy study of the atomic structure change of amorphous carbon films annealed in vacuum9(1991); http://dx.doi.org/10.1116/1.577160View Description Hide Description
Amorphous carbon films deposited at near room temperature on Si substrates by a direct current plasma chemical vapor deposition technique using CH4 gas have been annealed in vacuum. X‐ray photoelectron spectroscopy measurements show that, as the annealing temperature increases from room temperature to 300 °C, a significant shift (0.5 eV) of the C 1s electron binding energy is observed. The observed shift may be the indication of an atomic structure change of the amorphous films upon the thermal annealing. On the other hand, a large shift in the s p 2 bond peaks of the Raman spectra only occurs at annealing temperatures higher than 300 °C, which may be the indication of a structural change of the amorphous carbon films by the annealing. A two stage stress release mechanism is suggested: One for the atomic structure change and one for the film structure change.
Comparative study of the preparation of negative electron affinity GaAs photocathodes with O2 and with NF39(1991); http://dx.doi.org/10.1116/1.577161View Description Hide Description
We discuss the possibility of replacing oxygen with nitrogen trifluoride in the preparation of stable GaAs negative electron affinity (NEA) photocathodes. The different role of O2 versus NF3 exposure during the activation is examined by means of quantum yield and photocurrent lifetime measurements. Quantum yield results clearly demonstrate that NEA conditions can be attained also with NF3. Measurements performed on the same samples and in the same experimental conditions, show a remarkably different time evolution of the activation process in the two cases. However, as a result of a huge number of activations, no evidence of significantly larger lifetimes for NF3 prepared photocathodes can be deduced.
9(1991); http://dx.doi.org/10.1116/1.577162View Description Hide Description
We have investigated methods of chemical preparation and thermal treatments of InSb substrates to produce InSbsurfaces suitable for subsequent molecular beam epitaxy.Etching in a solution of lactic acid: HNO3:HCl was shown to produce an oxide surface layer which could be thermally desorbed. Growth on surfaces prepared in this manner produced precipitate‐free substrate–epilayer interfaces.
Measurements of catalytic efficiency of surfaces for the removal of atomic oxygen using NO* 2 continuum9(1991); http://dx.doi.org/10.1116/1.577163View Description Hide Description
The wall recombination coefficient (γ) of several materials for atomic oxygen was measured introducing a new method based on the NO* 2 continuum in a discharge‐flow system. This method has many advantages, such as the ability of measuring γ under different pressures, nonessentiality of measuring absolute atom concentrations, simplicity of experimental technique, etc. over the previous methods. Investigated materials in this work were selected specially considering the feasibility of industrial applications of plasmas. Although a few metals show slight differences, most of the calculated values of γ are in agreement with the literature values. Observed γ for Al and Al based materials were in the order of 10−3. The highest and lowest γ measured were of Cu and teflon, as 2.6×10−2 and 7.3×10−5 , respectively.