Film Contamination Research Articles

Comparison of Low-and High-Voltage SEM Images of Beryllium

Beryllium has a low secondary electron (SE) coefficient that produces a low signal-to-noise ratio, and in the past, this deficiency caused many workers to routinely coat beryllium with a high-Z material such as Au/Pd and examine the specimens at high voltage (HV). In principle, beryllium should be an ideal candidate for improved imaging with low voltage (LV), because its low atomic number permits excessive electron-beam penetration during HVSEM imaging — the use of LVSEM to analyze beryllium films was demonstrated previously. Unfortunately, low SE emission can seriously compromise LVSEM images from uncoated beryllium specimens, even with the improved LVSEM imaging capability of a SEM equipped with a field-emission gun (FESEM). This work demonstrates that some beryllium structures yield improved results with LVSEM imaging in a FESEM, particularly at low magnifications, while structures that require high magnification often yield better results with HVSEM.Specimens selected for this study include craters formed by a Cs+ ion beam in specimens that had been analyzed in an ion microanalyzer by secondary-ion mass spectroscopy (SIMS), machined surfaces, and fracture surfaces. SE images were obtained for this study with a Hitachi S-800 FESEM equipped with a cryogenic vacuum pumping system.Surface structure in a sputtered SIMS crater is shown at 2.0 kV in Fig. 1a and at 20 kV in Fig. 1b. This specimen was from commercial grade beryllium sheet. Prominent microtwins are visible in several grains in Fig. 1, and because of the excessive beam penetration at HV, the microtwins are better defined at LV in Fig. 1a than they are at HV in Fig. 1b. In addition, a fine faceted microstructure was formed by the ion beam that could be observed at intermediate magnifications, and it also was better defined at LV than at HV. The dark contamination film that is visible around some of the particles in Fig. 1a also is significant; this film was not apparent at voltages of 5 kV and higher, as demonstrated in Fig. 1b. SIMS ion images revealed that this material contained particles of several different compositions including carbides; since beryllium carbide hydrolyzes when exposed to moist air, the films are suspected to have formed by the decomposition of carbides when the specimen was removed from the ion microanalyzer after the SIMS analysis. This example demonstrates the excellent sensitivity of LVSEM to detect thin films, and it also suggests that contamination and the thin oxide film that forms on beryllium may impair SE imaging.

The Evolution of Silicon Wafer Cleaning Technology

The purity of wafer surfaces is an essential requisite for the successful fabrication of VLSI and ULSI silicon circuits. Wafer cleaning chemistry has remained essentially unchanged in the past 25 years and is based on hot alkaline and acidic hydrogen peroxide solutions, a process known as “RCA Standard Clean.” This is still the primary method used in the industry. What has changed is its implementation with optimized equipment: from simple immersion to centrifugal spraying, megasonic techniques, and enclosed system processing that allow simultaneous removal of both contaminant films and particles. Improvements in wafer drying by use of isopropanol vapor or by “slow‐pull” out of hot deionized water are being investigated. Several alternative cleaning methods are also being tested, including choline solutions, chemical vapor etching, and UV/ozone treatments. The evolution of silicon wafer cleaning processes and technology is traced and reviewed from the 1950s to August 1989.

A Study of CCl2F2 Magnetron Ion Etching Damage and Contamination Effects in Silicon

X-ray photoelectron spectroscopy has been used to evaluate Si surfaces exposed to magnetron plasma in CCl2F2 gas. Plasma exposure of Si surfaces results in a contamination film 12–33 Å thick and a damaged layer in the Si substrate. The contamination film consists of C–C, C–F, and C–Cl–F and/or F–F bondings. In addition, the C and F atoms penetrate into the Si substrate. On the other hand, the damaged layer consists of lattice defects and the contamination by the C and F atoms. Furthermore, from electrical measurements on Al/n-Si Schottky diodes, the damage depths are determined as a function of rf power. It is found that the damage depths correlate with the energy of ions impinging on the Si surface during plasma exposure.

A possible degeneration mechanism in stationary electrical contacts

A mechanism is proposed for the degeneration of stationary electrical contacts. The mechanism derives from W.W. Mullins' (1959) model of flattening of free surfaces through the action of capillarity forces. Material transport is assumed to occur by volume self-diffusion. Illustrative calculations of surface flattening and contact degeneration are presented for the case of Al. Calculations indicate that deformation rates increase with asperity curvature and are sensitive to activation energy for volume self-diffusion. These results suggest that the deformation of relatively small surface asperities (peak curvature of approximately 1 mu m/sup -1/ or larger) on defect-bearing Al is relatively rapid near room temperature and should be observable over a time interval of a few to several days. A simple electrical contact model suggests that the increase in contact resistance stemming from surface flattening in Al at room temperature can become noticeable in a time interval of a few days ( approximately 10/sup 6/ s). This result may explain the previously observed deterioration of contaminated Al/Al stationary electrical contacts operated at room temperature, after run times of 10/sup 5/-10/sup 6/. Since many metals are characterized both by rapid volume or surface diffusion and by a large surface energy, the results suggest that asperity flattening can lead to degeneration in all bulk electrical interfaces. Finally, since surface flattening leads to separation of contact surfaces, it makes it possible for fluid contaminant films to spread into the contact interface, suggesting that the mechanism of contaminant ingress proposed earlier as a primary cause of contact generation may instead be a consequence of surface operation in electrical junctions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Contamination induced stiction in drive level studies

When a thin liquid film of contamination is present between a head and disk, surface tension and viscous forces can increase the force that resists disk motion. A striction-force model is proposed. The model shows fairly good agreement with the data. Both the model and the data show that stiction forces increase with decreasing surface roughness and increasing contamination thickness. >

マグネトロンイオンエッチングによってＳｉ表面上に形成された汚染膜のＸ線光電子分光法による観察

X-ray photoelectron spectroscopy has been used to evaluate Si surfaces exposed to magnetron ion etching plasma in CCl2F2 gas. Plasma exposure of Si surfaces results in the deposition of a C, Cl, and F-containing film with the thickness of 10-13A. It was found that the surface of the contamination film consisted of C-Cl-F bondings, and that the inside consisted of C-F bondings.

Adhesion, friction and micromechanical properties of ceramics

The adhesion, friction and micromechanical properties of ceramics, both in monolithic and coating form, are reviewed. Ceramics are examined in contact with themselves, other harder materials and metals. For simplicity of discussion, the tribological properties of concern in the processes are separated into two parts. The first part discusses the pull-off force (adhesion) and the shear force required to break the interfacial junctions between contacting surfaces. The role of chemical bonding in adhesion and friction and the effects of surface contaminant films and temperature on tribological response with respect to adhesion and friction are considered. The second part deals with the abrasion of ceramics. Elastic, plastic and fracture behavior of ceramics in solid state contact are discussed. The scratch technique of determining the critical load needed to fracture interfacial adhesive bonds of ceramic deposited on substrates is also addressed.

Metal transfer in the sliding process

Metal transfer, during mutual sliding of steel and aluminum alloys, was studied using a model experiment. The model imitated sliding conditions which occur in metalworking operations: high load and short sliding distances. It allowed access to the aluminum-steel contact area without breaking the contact. It was found that the metal transfer rate is controlled by the area of the nascent steel surface which is in contact with the aluminum alloy. This area is proportional to the aluminum/steel hardness ratio and, as a result, the metal transfer is higher for the harder aluminum alloys than for the softer alloys. Based on the experimental results, it was suggested that higher roughness of the steel surface promotes metal transfer in part because the rougher surface has a larger number of sharp asperities, which deform plastically, causing the rupture of oxides and other contaminant films and exposing nascent steel, and in part because of increased ploughing and mechanical interlocking. Effects of surface segregation of the alloying elements and of mechanical adhesion on metal transfer are also discussed.

Surface effects of satellite material outgassing products

Abstract : Optical property measurements of condensed contaminant films on cryogenically cooled substrates were completed for 18 additional materials to those reported in AEDC-TR-87-8, 'Surface Effects of Satellite Outgassing Products'. The materials studied were either being considered for use in space or in ground test chambers at Arnold Engineering Development Center (AEDC). Infrared spectral transmittance measurements were made for thin films of contaminant deposited on a 77 K germanium window under vacuum conditions (. 000001 torr). Material temperatures were controlled at 125 C for 24 hr to provide the outgassing source. The objective of this report is to present the refractive (n) and absorptive (k) indices of the condensed outgassing products of satellite materials which have been determined from the experimental measurements of the infrared transmittance spectra.

X-ray Diffraction of Thin Oxide Films on Soldered Module Pins

AbstractThe presence of metal oxide films from wave solder baths on timed module pins are partly responsible for non-wet problems in subsequent soldering steps. The cylindrical geometry of the pins lends itself to the characterization of thin oxide films by using the highly sensitive Debye-Scherrer camera method. As confirmed by Electron Hicroprobe Analysis (EMA), pins containing thin oxide films were used to obtain the diffraction patterns. A software program was developed that subtracts the diffraction angles of an oxids-free control pin from the pattern of the contaminated pin, and tabulates the residual d-spacing (interplanar distance) of the contaminant film.

A Simple Method for Characterizing the Bondability of Metallization Surfaces

A probing technique is described which measures breakdown voltages of contaminating films on metal surfaces. These breakdown voltages are correlated with the bondability and wettability of the surfaces. The probe setup consists of a fine gold wire with a ball terminal and a tungsten whisker. Three examples are described. The breakdown voltage on Al metallization on a Si chip and on plated Ag film on a leadframe both correspond to the ease of Au wire bonding. In contrast, the Au-Si eutectic layer demonstrating a large breakdown voltage features poor solderability and low bonding reliability. The An ball probe is confirmed to be a convenient tool for quality control in thin-film manufacturing and device assembly processes.

Secondary effects of single crystalline silicon deep- trench etching in a chlorine-containing plasma for 3- dimensional capacitor cells

The main problems occurring during the trench etching in a chlorine containing RIE process are considered: (1) deposition of contaminating films on the trench sidewalls, (2) micro masking effects of those contaminations leading to the so called “black silicom”, and (3) “trenching effects” on the bottom of the trench cell. By means of SIMS and Auger measurements it is shown that the contaminating films on the trench walls are formed by unsaturated B xCl y species from the BCl 3 plasma. By the same analytical methods it is obtained that the so called black silicon occurring in a pure Cl 2 plasma is caused by micro masking by Al-, Si- oxide clusters which stem from surface adsorbed oxygen and the alumina covered cathode. Furthermore it is observed that the trenching effect occurring with Cl 2 plasma can be significantly reduced by adding BCl 3. BCl 3 reduces the number of Cl + 2 ions and therefore favours the formation of passivating species on the surface to be etched. This passivating layer in return reduces the trench formation at the bottom.

Bond Strength in Cold Roll Bonding

The mechanisms producing metallic bonds in cold welding are fracture of the contaminant surface film, extrusion of base material through the cracks and builòup of real contact and coalescing with base material of the opposite surface. Bay has earlier proposed a general model for the bond strength in cold welding based upon a continuum mechanic analysis of the local surface deformation involving these mechanisms. Basic parameters are surface expansion, normal pressure and two parameters describing the physical properties of the contaminant surface film. In the present paper this model is developed for cold roll bonding. Using the slab method and a friction model developed by Wanheim and Bay, the pressure distribution in the roll gap is estimated. This calculation is introduced when calculating the bond strength. The influence of roll radius to plate thickness ratio, friction, surface preparation and reduction in rolling on the bond strength obtained in Al-Al cold roll bonding has been calculated. The accordance with experimental measurements of the bond strength is satisfactory.

Contact Resistance on Surfaces with Nonuniform Contaminant Films

A probe has been designed for making point-by-point mapping measurements of contact resistance over large surface areas. The probe, which operates in the low force range of 5 mg to 5 g, permits simultaneous measurement of both contact resistance as a function of load and the integral of contact resistance versus force. To accomplish this a magnetically damped torsion bar mounted pendulum/sample holder is electromagnetically driven against a smooth spherical probe tip mounted directly on a force transducer which, in turn, is mounted on an x-y precision translation stage. The associated instrument package includes a variable sweep rate electromagnetic drive, high-frequency rejecting load and contact voltage amplifiers (the latter being an ultrastable differential amplifier), a voltage-to-frequency converter, a scaler, and an x-y recorder. The probe has been used to explore spatial variations in contact resistance due to a nonuniform fiuoropolymer film on a gold substrate. The data show a large statistical variability, which is simply analyzed by assuming normally distributed surface asperities and applying this assumption to the accepted model for multi-asperity electrical contact. This procedure leads to a prediction that the square of the conductance will follow a Gaussian distribution. Contact resistances that were obtained in the course of this work as a function of force are shown to be consistent with the multi-asperity model, Data are presented which agree well with the statistical model. This insight into the statistical variability of contact resistance data is useful in that it suggests a successful data smoothing technique that presents a reasonable appearing line map of the effective contact resistance due to a nonuniform film. It is demonstrated that data smoothed in this fashion can be favorably compared to the degree of surface contamination as determined with a scanning electron microscope.

Slow contamination and fast surface state effects SEM studies of Si phototransistors

The rate of growth of contamination films on the surface of Si phototransistors under examination in a scanning electron microscope was measured by multiple beam interferometry. The contamination film reduced the barrier electron voltaic effect current (i.e. the ″EBIC″ signal) by 10 to 20% after scanning a single line for an hour. The photovoltaic effect current of the transistors however was reduced by 90% after scanning the whole area of these devices for a few seconds. This is ascribed to the fact that bombardment-induced changes in surface electrical conditions strongly affect the carriers produced near the surface by photons but not the carriers produced at depths up to 2μm or more by the electron beam.

Field desorption and field ion surface studies of samples exposed to the plasmas of PLT and ISX

Modifications to the surface of field-ion specimens exposed to plasma discharges in PLT and ISX determined by Imaging Atom-Probe, Field Ion Microscope, and Transmission Electron Microscope analyses have in the past shown several consistent features. Surface films consisting primarily of limiter material with trapped plasma and impurity species have been found to reside on samples with direct line of sight exposure to the plasma during the discharges. Control specimens placed in the tokamak, but shielded from the plasma, on the other hand, remained free of deposits. When exposed to only high power plasma discharges, samples placed at the wall position in PLT and ISX have survived the exposures with no evidence of damage or implantation. In this paper we describe the results of a recent exposure in PLT in which for the first time samples of stainless steel were included for High-Field Surface Analysis. Tokamak operating conditions, including stainless steel limiters, titanium gettering between discharges, and the occurrence of a disruption, also distinguished this exposure from those carried out previously. Even with stainless steel limiters, contaminant films were found to be deposited on the samples at a rate (50–60 Å in six shots) comparable to earlier exposures with carbon limiters (500 Å in 69 shots).

Effect of substrate surface conditions and impact velocity of sprayed particles on coatings produced by plasma spraying

In high temperature plasma spraying, surface conditions of the substrate and the impact velocity of sprayed particles are particularly important in the consideration of factors influencing the resultant coatings. In this study an induction plasma torch was used to spray aluminum, tungsten and nickel particles onto flat (111) single-crystal silicon substrates with different surface preparations. In addition to experiments with silicon substrates with clean surfaces and substrates with previously evaporated aluminum over oxide coatings, oxide films and water vapor, hydraulic oil and grease contaminant films were used to investigate the influence of these surface conditions on adhesion formation. The impact velocity of the sprayed particles was controlled by controlling the speed of a centrifugal disc on which the substrate was installed to traverse the plasma effluent. Scanning electron microscopy was used extensively to characterize features of the particle-substrate interaction and to compare the individual behaviors of the metal particles used. Microcavitations and dislocations resulting from the plastic deformation of the substrate were revealed by the successive use of a metal and a special dislocation etchant.

Surface analysis of field-ion samples exposed to the plasma of the impurities studies experiment (ISX-A) tokamak

Measurements of the modifications to the surface of field-emitter samples (tips) exposed to plasma discharges in the impurities studies experiment (ISX-A) tokamak have been carried out for the first time in order to help further identify the physical processes occurring at the tokamak wall during machine operation. The specimen tips were exposed to 40 high-power hydrogen discharges in ISX-A at a position 5.0 cm back from the plasma edge defined by carbon and molybdenum limiters. In contrast to results obtained on silicon samples exposed at a similar location in ISX-A using Rutherford ion backscattering analysis, field-ion images of tungsten tips taken before and after exposure indicate that no structural rearrangement of the specimen near-surface region occurred, even on an atomic scale. Contaminant films 60±10 Å thick, deposited on the specimen surfaces during exposure, were observed in the transmission electron microscope. Control tips, placed in the tokamak during the discharges but shielded from direct plasma exposure, remained free of such deposits. Imaging atom-probe depth profiles into the exposed sample substrates revealed that neither plasma nor impurity species were implanted beyond the deposited surface films. The results from this study are compared with those from earlier analyses of samples exposed in the Princeton large torus (PLT) using similar techniques and also with the results of analyses of macroscopic samples exposed at the same ISX-A location using other surface-sensitive techniques.

Contact Resistance Characteristics at Low Temperature

The application of cryogenics to the fields of electrical engineering has great possibility. However, performance of contacts used in the application may be estimated to deteriorate due to contaminant films. It is important subject to clarify the electrical conduction mechanisms of contacts covered with contamination films to obtain low contact resistance characteristics at the low temperature as well as at the normal temperature. From these standpoints, electrical conduction mechanism and contact resistance at low temperature were examined. Schottky current was observed in oxide film covered contacts over a range from 300 K to 77 K. The contact resistance at the low temperature increased by approximately 100 times or more over that of normal temperature due to Schottky conduction. For clean surface, the contact resistance passes through a minimum at low temperature due to thermal contraction of true contact area. In conclusion, Schottky conduction was found predominantly. Therefore, if the contaminant films exist at the contact surface, contact resistance increased remarkably at low temperature. Moreover, even for contacts of clean surfaces, contact resistance has a limiting value unlike resistance of metal conductors. These results give a serious problems in the performance of contacts for the cryogenic application.

Kinetics of Thermocompression Bonding to Organic Contaminated Gold Surfaces

The kinetics of thermocompression gold ball bonding were studied for gold metallization contaminated with organic films. The contaminants studied were residual photoresist and atmospheric contamination. For gold metallization contaminated with organic films, thermo-compression gold bonding is a two-stage process. The first stage occurs within a fraction of a second and results from the mechanical disruption of barrier films by shear displacements at the faying surface. Bonding during this initial stage is a strong function of the interface temperature, the bonding force, and the contaminant films. The second stage involves growth of the metal-metal interfaces by a sintering phenomenon. Growth of bonds by sintering also occurs in the absence of external loads though at a decreased rate. Analysis of the rate data leads to the conclusion that the growth mechanism follows a parabolic rate law and, in the absence of external loads, exhibits an activation energy of approximately 11 kcal/mole. Under high external loads, the rate of second-stage growth exhibited little temperature dependence, indicative of a stress-assisted process. Bond growth rates for the atmospherically contaminated metallization were an order of magnitude higher than that for the photoresist contaminated samples.