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4 Characterization Methods of Atmospheric Plasma Surface Modifications

4.1 Surface Characterization Techniques

All solid surfaces form an interface. Potential interfaces include solid-liquid, solid-gas or solid-solid, dependent upon the existing state. There can also be a first state of matter - fourth state of matter interface (solid-plasma) if a solid substrate is situated interfacially with a plasma.

As described in the previous section, there are a number of sur-face properties which can be created through polymer depositions, atmospheric plasma surface modification, surface corrosion resis-tance, surface water resistance, surface catalysis, and many others which require specific types of surface characterization techniques to examine and understand the potential of their interfacial effects. Targeted modifications in surface properties such as improved wet-tability, chemical bonding, and increased antireflective effects are key to commercial implementations within aerospace, nanotech-nology, nuclear science, solar technology, electronics, and industrial manufacturing.

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Atmospheric Pressure Plasma for Surface Modification by Rory A. Wolf

Copyright © 2013 Scrivener Publishing LLC

82 ATMOSPHERIC PRESSURE PLASMA FOR SURFACE MODIFICATION

There have been ongoing efforts underway to study surface prop-erty changes as they relate to various atmospheric plasma surface modifications. However, there is a need for a deeper understand-ing of atmospheric plasma surface effects as this field of surface modification accelerates in use, and most particularly beyond the molecular level to atomic and nano scales. This is because there are commercial needs for increasingly controlled processing outcomes to transition manufacturing processes from in-chamber batch pro-cesses. Surface modifications in chamber such as sputtering and ion implantation currently effect bond strength, substrate composition, and topographical changes, all of which are outcomes dependent upon the substrate's material characteristics and its reaction to this type of plasma processing. The characterization of these nanoscale modifications to metal surfaces, for example, is usually accom-plished by techniques such as X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM). The implantation of nitrogen into metals to improve their surface hardness proper-ties and surface wear resistance is routine, whereby the base sub-strate is exposed to a nitrogen-based plasma at high pulse rates. This implants nitrogen ions into the metal surface, but also simulta-neously increases its surface temperature. This increased tempera-ture will also allow for progressively better surface diffusion of the implanted ions at about a depth of up to several microns. It is pre-cisely these diffusion characteristics which require surface charac-terization techniques to understand the modification mechanisms and possible plasma process improvements. This characterization also provides details as to the form of modification which has taken place at up to nanometer depths. And XPS analysis is the primary tool to be applied since it can characterize these shallow depths, it has the ability to identify chemical functionalizations, and identify other surface properties. It is effectively used to profile treatment depths of thin layer depositions, as well as providing bonding pro-files. The addition of SEM analysis provides information on changes in surface topography and surface morphology. From this point, we will analyze each surface characterization technique with respect to function and performance for atmospheric plasma processing.

4.2 X-Ray Photoelectron Spectroscopy (XPS)

X-ray Photoelectron Spectroscopy is a highly sensitive surface anal-ysis technique useful in characterizing solid materials at a depth

CHARACTERIZATION METHODS 83

of approximately ten nanometers. The technique provides both the elemental composition of the surface, but also the binding energy states of these elements. The wide diagnostic range of this technique has lead to its routine use with all plasma-related surface modifica-tions. Following an introduction to the basic capabilities of the tech-nique, we will examine how its abilities provide new insights in the field of plasma surface characterization.

4.2.1 Des ign and Analytical Capabilities

X-ray Photoelectron Spectroscopy (XPS) is also known as Electron Spectroscopy for Chemical Analysis (ESCA). It is applied to many surface modification outcomes discussed previously such as polym-erization, surface corrosion, metal sciences, biomedical applica-tions, microelectronics, and others. There are new developments in XPS analytical systems which compensate for surface charge with insulative substrates. Additional developments in these types of detection capabilities will further broaden the usefulness of XPS and the imaging of XPS.

The principle of XPS is based on an X-ray induced emission of electrons which are characterized according to their kinetic energy. This is known as the "photoelectric effect" whereby photoelec-trons are ejected from the substrate surface through the excitation of X-ray photons and characterized within an electron energy ana-lyzer. Elemental kinetic energies and their binding (oxidative) states are examined. With the exception of helium and hydrogen, all of the elements can be detected by XPS, When an X-ray quant strikes a surface atom, electrons from the inner shells of the electron lev-els are emitted. Their kinetic energy will be dependent upon the energy of the impacting photon, the binding energy of the electrons, and the ambient pressure level. Within these conditions, it is only the outer layers of a surface from which electrons can escape with any loss of energy. Because of this, the measurement depth of XPS is limited to fifty atomic layers (ten nanometers).

Although XPS analytical tools vary in manufactured design and capabilities, there are common and essential components to all sys-tems, namely:

• Excitation Source: X-rays are generated by electrons which are removed from a heated filament, accelerated, and directed to impact the target anode. Magnesium and aluminium are typically utilized as anodes in


X-ray sources. The photons pass through a monochro-mator to eliminate high energy radiation and other species to clean the spectra of satellite lines to pro-duce an improved signal/noise ratio for more accurate interpretation. In addition, there is a reduction in "full width at half maximum" of the excited radiation which results in improved energy resolution and better defi-nition between the different species.

• Electron Energy Analyzer: This analyzer is based upon the deflection of charged particles relative to their kinetic energy within an electrostatic field. Once these particles pass through an energy-selecting field, only the electrons are detected at a velocity within a specific kinetic energy range. To acquire the entire spectrum, the entire target region is scanned in a successive method by continuously changing the deflection potentials. New detectors, known as delay line detectors, utilize a multi-channel plate stack above a delay-line anode. There are three modes of operation - scanned spectroscopy (high energy resolution), unscanned spectroscopy (high speed), and 2D imaging mode (high special resolution).

• Ultra-High Vacuum (UHV) System: The use of a vac-uum is critically important for this technique. Since there is low analytical surface depth, any contamination layer on the substrate surface will distort results. Once elec-trons are ejected from the substrate surface, the remain-ing gas pressure must be low so as not to decelerate the electrons with residual gas molecules. Vacuum compo-nents which use heated filaments and high voltage to form electrons require ultra-high vacuum conditions.

• Data Processing Systems: These are required for effi-cient processing of highly complex acquired data, and for analysis of the spectra.

• Ancillary Processing Features: Key systematic process-ing features available from most manufacturers include adjustable fixturing devices for holding substrates at special angles to modify electron emission depth for specific measures, ion guns for surface cleaning and depth profiling, surface heating devices for analyzing surface decompositions, and additional chambers for analyzing gas-specific reactions or plasma treatments.


4.2.2 Application Examples

Polymers are routinely analyzed by XPS techniques to determine their elemental composition before and after plasma surface modi-fication. A spectrum of untreated polyethylene (PE) will include a carbon Is line at a binding energy of 285eV (near theoretical val-ues), but will not show another line since hydrogen is not detect-able. Therefore, hydrogen is not considered in the composition, causing the composition determination to be 100 atomic percent carbon. For polyester, the elemental composition will show values of 71.4 atomic percent carbon, and 28.6 atomic percent oxygen (near theoretical values).

From a chemical composition standpoint, high resolution ele-mental spectra can profile a single line for aliphatic carbon (C-C, C-H) at a binding energy of 285.0 eV. For polyester, a spectrum of aromatic carbon at 285.0 eV, a carbon - oxygen bonded carbon (C-O-C) group at a binding energy of 286.6 eV, and a carboxylic carbon (0-C=0) at 289.0 eV will be found. The oxygen spectrum will also detect carbonyl oxygen and oxygen in the C-O-C group.

4.2.3 Imaging of XPS

With XPS imaging, microstructured surface analysis can be explored. XPS imaging is a useful method for determining the distribution of chemical species across a sample surface. Very early methodologies for this technique involved the use of XPS mapping whereby an X-ray probe is scanned across a sample surface and the surface image is developed pixel by pixel as the analysis spot is transitioned across the sample. There are other methodologies whereby parallel detec-tion of a defined field of view over a specific range of binding energy is possible. Further developments in detector design have lead to quantitative methods of counting pulses which can provide high res-olution XPS images with quantitatively-defined levels of intensity.

The initial step with XPS imaging analysis is to map the different chemical elements present on the substrate of interest. The primary principle of this imaging is that a full XPS spectrum of species is collected at each pixel of an image. Imaging software enables the extraction of spectra of areas being characterized and the recon-struction of an image of a chemical state by chemical mapping. By this method, images can be transformed so they are useful and chemically contributory.


294 292 290 288 286 284 282

Binding energy (eV)

Figure 4.1 Photoemission study of fluorination atmospheric pressure plasma processing on EPDM.

4.2.4 Element Mapping

Elemental imaging requires that the detector be set to a fixed energy level, and that successive imaging of elements (or binding states) takes place. A characteristic X-ray intensity is measured relative to the position chosen on the sample. Using variations in the intensity of the X-ray at any energy value indicates the concentration of the focus element across the substrate surface. Typically, multiple maps are simultaneously recorded utilizing image intensity as a function of the local concentration of the element(s) identified as present. A resolution of approximately one micron resolution is possible.

4.3 Static Secondary Ion Mass Spectrometry by Time-of-Flight (ToF-SIMS)

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is a surface analysis method that uses a pulsed ion beam to displace molecules from the outermost surface of the sample. The particles are displaced from the atomic mono-layers on the substrate sur-face, known as secondary ions. These particles are then transported


into a "flight tube" and their mass is measured by calibrating the time at which they reach the detector. This represents their time of flight. There are three modes of operation using ToF-SIMS: 1) sur-face spectroscopy, 2) surface imaging, and 3) depth profiling.

ToF-SIMS is frequently referred to as "static" SIMS because a low primary ion current is used to "brush" the substrate sur-face to release ions and molecules for analysis. A method known as "dynamic" SIMS contrasts this insofar as it is the preferred method for quantitative analysis because a higher primary ion current yields a faster sputtering rate and produces a significantly higher ion yield. Therefore, dynamic SIMS will create better data for measured trace elements. However, organic compounds are destroyed by dynamic SIMS, and no diagnostic information can be obtained.

ToF-SIMS is capable of analyzing the mass resolution in atomic mass units. Those species with the same nominal atomic mass units, such as Si and C2H4, are still distinguishable since it is known that there is a shift in mass as atoms enter a charge-bound state. ToF-SIMS will measure atomic mass in a range between 0 and 10,000 atomic mass units, positive and negative ions, molecular compounds and isotopes. Elements in the parts-per-million range can be detected, and submicron images can be mapped. Profiling of surface depth is available through ToF-SIMS and chemical analyses can take place following in-chamber sputtering. Pixels with map-ping by ToF-SIMS will represent the full spectrum of a mass. This allows for the production of mappings for any desired mass, and the subsequent analysis of certain surface regions to examine their chemical makeup following computer processing.

To characterize the process on a second level, ToF-SIMS utilizes a pulsating particle beam of cesium or gallium to dislodge vari-ous chemical species on a substrate surface. Normally, the parti-cles dislodged at the impact location are positive or negative ions. Particles removed beyond this level are molecular in size, such as organic compound fragments. These particles are accelerated back toward the detector and their time of flight measured, typically in nanoseconds. Resolutions in the 0.00 x atomic mass units, or one part per thousand of a photon's mass, is possible. ToF-SIMS instru-mentation will typically be fitted with an ultra-high vacuum sys-tem to allow for acceleration of the mean free path of the positive and negative ions released to the flight path, a particle "gun," a circularly-designed flight path which uses electrostatic analyzers


to target the Cesium or Gallium particle beam, and an atomic mass detector system. Measurable outcomes from analytics associated with ToF-SIMS include:

• Mass spectrum survey of all atomic masses from 0-10,000 amu

• Rastered beam maps of masses on a submicron scale • Depth profiling by removal of surface layers through

ion beam sputtering

ToF-SIMS instrumentation is typically equipped with highly powerful hardware and software designs for surface analysis. Leveraging this architecture, a key feature of ToF-SIMS software is its ability to provide quantifications for what is known as "ret-rospective" analysis. With this analysis, every molecule in the selected sample detected by the system can be effectively stored as a function of its molecular mass and from where it originated. This allows for the generation of chemical mapping (spectra mapping) of sample regions which were not previously identified after the original data was collected.

Regarding applications, ToF-SIMS is primarily used in studying the surfaces of materials such as polymers, engineered pharmaceu-ticals, and electronic semiconductor materials. Principally how-ever, ToF-SIMS can be applied to any surface modifying reaction such as reduction, catalysis, precipitation, and sorption. Recently, ToF-SIMS has been applied to applications pertaining to geological work involving the identification of organic films and biomarkers in mineral deposits, characterization of organic molecules in coal, and the analysis of metals and other particles composing magma and interplanetary dust to name a few.

As a high-end surface analysis technique, ToF-SIMS has a num-ber of definable advantages and disadvantages which can be sum-marized as follows:

Advantages

Surveys all masses discovered on material surfaces, includ-ing positive and negative ions, molecular level compounds, isotopes

Disadvantages

Data-overload, insofar as every image pixel will also contain a full mass spectrum, requir-ing extended time periods to analyze a data set


Maps elements and chemical spe-cies at submicron level

Distinguishes species having similar mass with high mass resolutions >0.00 x atomic mass units

Sensitive to ppm and ppb for detecting trace elements and compounds

Depth profiles from atomic layers to 10s of nanometers

Ability to analyze both conduc-tive and nonconductive sub-strate samples

Provides analysis which is not destructive

Provides retrospective analy-sis for spectra and imagery interpretation

Limited optical profiling capa-bilities for surface regions of interest

Imparting of surface charges may occur if routines are not followed

Will not provide highly-quan-titative analyses of surveyed masses.

Requires purposeful data collec-tion and focused analyzing/ interpretation techniques due to the number of data sets required

Image-shifting is possible when collection mode shifts from positive to negative ion data on the same spot

4.4 Atomic Force Microscopy

Atomic Force Microscopy (AFM) is also known as scanning force microscopy (SFM) and is a high-resolution surface scanning tech-nique with a resolution less than a nanometer, which is over one-thousand times greater than optical diffraction limits. Although Binnig, Quate, and Gerber innovated the atomic force microscope in 1985, the first commercial atomic force microscope was intro-duced in 1989. The atomic force microscope was initially developed to overcome a disadvantage with other techniques which could only image conductive or semi-conductive substrate surfaces. AFM could image nearly any type of surface, including polymers, glass, composites and biologically-rendered samples. The original AFM


was composed of a diamond fragment attached to a strip of gold foil. The diamond tip made contact directly with the substrate surface, with van der Waals forces providing the interaction mechanism.

AFM is one of the best tools for imaging, profiling and measuring nanoscale surface species. The information is gathered scanning the surface with a mechanical probe. Piezoelectric elements that control tiny but accurate and precise movements with electrically-actuated commands enable the very precise scanning. There are also varia-tions of AFM whereby electrical potential can be scanned using conductive cantilevers. More advanced variations of AFM allow currents to pass through the tip to probe the electrical conductivity (current transport) of the surface.

AFM technology utilizes a cantilever with a probe at the end that is used to scan the substrate surface. The cantilever is typically manufactured with silicon nitride or only silicon. The probe has a radius at its tip of several nanometers. As the probe is positioned near the sample surface, several types of forces occur between the probe tip and the substrate sample which leads to a deflection of the cantilever. Depending upon the sample protocol, the types of forces that are measured in AFM include: 1) mechanical contact force, 2) van der Walls forces, 3) capillary-type forces, 4) chemical bonding forces, 5) electrostatic forces, 6) electromagnetic forces, 7) Casimir forces, 8) dissolution forces, and others. In addition to forces, other factors can be simultaneously measured by the use of specialized scanning microscopy and thermal photospectromicroscopies. It is typical that the deflection is measured using a laser whose spot is reflected from the top surface of the cantilever into a series of pho-todiodes. Additional methods which are used include optical inter-ferometry capacitative sensing, or piezoresistive AFM cantilevers. These cantilevers are manufactured with piezoresistive elements which function as a strain gauge. Strain in the AFM cantilever caused by deflection can be measured best by laser deflection or interferometry.

The positioning of the probe is critical to AFM accuracy. With the probe scanned at a fixed height, there would be a possibility that the probe would collide with the substrate, creating damage to the surface. For this reason, a sensor mechanism is used to modify the distance of the probe to the sample surface to maintain a consis-tent force between the probe and the substrate. Typically, the sub-strate is positioned on a piezoelectric fixture that can manipulate the substrate in the z direction to sustain a consistent force, and


the x and y directions when scanning the substrate. An alternative methodology utilizes a tripod-type orientation whereby three piezo crystals are used for scanning the x, y and z directions. With recent designs, the probe is mounted to a vertical piezo scanner as the substrate is scanned in x and y directions using an additional piezo block. The final mapping of the area represents the topographical scan of the substrate. In the current era, AFM uses a laser beam deflection system which was introduced by Meyer and Amer. With this methodology, a laser is deflected from a reflective AFM lever and toward a position-sensing detector. AFM probes and cantile-vers are manufactured from silicon or silicon nitride.

AFM is operable in a number of modes, depending upon the required application. Generally speaking, the available imaging modes are separated into static (contact) modes and dynamic (non-contact) modes.

4.4.1 Static Mode

This mode is the primary mode of operation and widely used. As the probe is scanned over the substrate surface, it is deflected by the substrate's undulating surface. With the mode set for con-stant force, the probe is continually adjusted while maintaining a constant distance above the substrate so as to maintain consistent

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OLDPE O Ar/LDPE □ (2min) F& AR/LDPE Δ (5min) FC/AFVLDPE O {10min) FC/AP./LDPE

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Figure 4.2 Nanoscale mechanical and tribological properties of fluorocarbon films grafted to plasma-treated LDPE film.


deflections. The data recorded is relative to the adjustment made. But the tracking over the surface by this method is limited by the feedback circuit. It is not uncommon for the probe to be allowed to scan surfaces without adjustment whereby only deflection is mea-sured. This approach is known as variable deflection mode and is used for high speed, atomic level scans. Since the probe is in inti-mate contact with the substrate surface, the lever stiffness must be less than approximately 5nM/nm, the effective spring constant which holds atoms together.

4.4.2 Dynamic Mode

Dynamic mode uses an oscillating cantilever and is therefore classi-fied among AC modes. In this mode, a rigid cantilever is oscillated whereby the probe is in close proximity with (but not touching) the substrate. The forces between the probe and the substrate surface are very low. This type of detection measures changes in the ampli-tude (resonant frequency) of the cantilever.

A variation of dynamic mode is known as Dynamic Force (inter-mittent-contact) Tapping Mode AFM. It is most commonly referred to as tapping mode, intermittent-contact, or generally as dynamic force mode (DFM). A rigid cantilever is oscillated closer to the substrate surface than in noncontact mode. A portion of the oscillation operates in a mode whereby the tip intermittently "taps" the substrate surface. Quite rigid cantilevers will typically be used, since the probe has the possibility of getting mired within the contamination layer. However, the primary advantage of tapping the substrate surface is better lat-eral resolution with soft samples. And further advantages are real-ized where species are not readily adsorbed on a substrate surface.

Another methodology of AFM is force modulation. Force mod-ulation is a method used to examine the properties of materials through interactions between the probe and substrate. Either probe of substrate is oscillated at high frequency. The slope measurement of the force versus distance curve is measured and then correlated to the elasticity of the substrate. The resulting data is acquired, as is the topography, so that a comparison can be made of the sample's height and material properties.

With regard to AFM imaging, phase mode imaging features a phase shift of the oscillating cantilever relative to the driving sig-nal and it is this shift which is measured. This phase shift is cor-related with substrate properties that effect the probe and substrate


interaction. This phase shift is used to distinguish areas on a substrate sample among properties such as adhesion, friction and elasticity. The technique is commonly used simultaneously with Dynamic Force Mode, so that surface topography can also be measured.

While most surfaces modified by plasma technologies can be readily profiled by AFM, applications which present more com-plexity in analyzing are three-dimensional surfaces where mor-phological and topographical surface modifications are induced by plasma processing techniques such as cold plasmas. One example of these surfaces is a natural or polymeric structure. Surface effects can certainly be analyzed across different plasma exposure times. A typical result which can be expected is progressive degradation of the surface with increasing surface roughness, where the modifica-tion can be ascribed to a plasma-based physical process.

However, it can be difficult to judge in advance if a contact (non-tapping) mode can be applied for characterization of a particular sample. As such, measurements of probe-to-sample forces are per-formed with deflection vs. distance force curves. This examination of curves with probes of various stiffness can be useful to define operating parameters and the most appropriate probes to be used for imaging. These force curves can assist in recognizing the capil-lary forces and adhesion involved, in determining those regions of attractive and repulsive probe-to-sample forces, in determining the range of deflections with probe-to-sample elastic deformation, and for determining a probe-induced sample deformation.

In addition to force curves, there are other more practical proce-dures to insure reliable and controlled imaging. Following probe engagement, the operator can attempt lower deflection set points in an effort to minimize the probe force, or simply increase the feed-back gains and further optimize the scanning rate to stabilize the imaging. This can be accomplished when height contours in the trace and retrace directions match each other and topographic or height images are practically identical in consecutive scans [25].

Realistically, the topography which is observed may not nec-essarily be the top sample layer. Rather, what may occur within the image represents a more rigid sublayer when the top layer is removed by the probe. This theory can be verified by initiating a wider scan at the same probe-force conditions. During the scan, possible sample damage can be related with the time the probe is spent in the sample location. Therefore, a probe-damaged area is often considered a "window" within the larger scan. If this effect is


discovered, a lower probe-force or a softer probe should be used for imaging the top layer.

If a sample substrate will allow for imaging at different probe-sample forces, such a circumstance can be leveraged for "compo-sitional" imaging within the contact mode. Areas at the surfaces which have different stiffness can be depressed by the probe-force to various levels. As such, contrasts in height or topographical images will therefore depend on the imaging force.

4.5 Scanning Electron Microscopy

Scanning electron microscope (SEM) utilizes a beam of focused high energy electrons to generate signals at the surface of solid substrate samples. The signals that originate from interactions between these electrons and substrate surfaces expose properties about the substrate such as surface morphology, chemical composi-tion, and the molecular orientation (including crystallinity) which composes the sample. Typical with most applications, profiling data is collected over a specified area of the sample surface which is processed to generate a two-dimensional image to display varia-tions in the aforementioned properties. Sample areas range from approximately five microns to one centimeter in width and can be imaged in scanning mode utilizing standard SEM techniques. SEM magnifications can range from 20-30,000 and with a resolution of between 50 to 100 nanometers. SEM can also be used to analyze point-specific locations on the substrate sample. This approach is particularly used in qualitatively profiling the chemical composi-tions of the substrate surface, its crystallinity, and the orientations of its crystals. SEM is very similar in design and function to electron probe microanalysis (EPMA), a nondestructive technique for quali-tative and quantitative elemental analysis of micron-sized volumes at surfaces, with sensitivity at the level of ppm..

Principally, electrons which are accelerated in the process of SEM carry with them a massive amount of kinetic energy which is gradu-ally dissipated into various signals which are produced by interac-tions between the substrate surface and these electrons when they are decelerating in the solid substrate's surface. These signals will include secondary electrons which will produce the SEM images, backscattered electrons, diffracted backscattered electrons, photons, and visible light. As alluded to previously, the secondary electrons


and backscattered electrons are most commonly utilized for imag-ing samples. Among these, secondary electrons are the most useful for displaying morphology and topography on samples, and back-scattered electrons are most useful for displaying compositional contrasts in what are known as multiphase samples. When incident electrons collide with electrons which are in orbital shells of atoms of the substrate, X-rays are produced. When these electrons return to lower energy states, their X-rays will have a fixed wavelength which corresponds to the difference in energy levels of electrons in different orbital shells for a given element. When this occurs, char-acteristic X-rays are generated for each element of a mineral that becomes excited by the SEM electron beam. In the context of this process, SEM analysis is therefore nondestructive in that the X-rays created by the electron interactions will not lead to a loss in sub-strate volume. Therefore, any substrate can be analyzed repeatedly.

There are a number of critical components which comprise most SEM systems. These include the following:

• Source of electrons (electron "gun") o Thermionic - apply thermal energy to a tungsten to

drive electrons toward the specimen o Field Emission - create a strong electrical field to

draw electrons away from their associated atoms • Condenser Electron Lenses - focus and control the

electron beam • Sample Stage - stabilize and manipulate the specimen • Detectors

o Everhart-Thornley - register secondary electrons o Backsca t te red Elect ron - def ine s u b s t r a t e

composition o X-ray - also define substrate composition

• Display/Data Output Devices • Installation/Utility Requirements

o Power Supply o Vacuum Chamber o Cooling System o Vibration Protection o Ambient Magnetic/Electric Field Protection

SEM systems will always have at least one detector, usually a Everhart-Thornley secondary electron detector. As the electron


beam passes over the substrate surface, it physically interacts with the surface of the substrate, dislodging secondary electrons in unique patterns. A secondary electron detector attracts these scat-tered electrons and, depending upon the number of electrons that meet the detector, registers different levels of brightness on a moni-tor. The capabilities of a particular SEM instrument are, of course, dependent upon the type of detectors it utilizes.

SEM is leveraged to generate high-resolution dimensional images of substrate surface shapes, as well as to display spatial changes in surface chemical compositions, by: 1) obtaining elemental maps or chemical analyses, by 2) differentiation of phases based on mean atomic number/relative density, and by 3) elemental composition maps which define differences in element activators. SEM is also typi-cally utilized to identify phases based on chemical analyses and crys-tallinity measures. AS mentioned above and worth repeating here, accurate measurement of surface features to 50 nm can be accom-plished by SEM. Backscattered electron imagery can be leveraged for phase discrimination in multiphase substrate surfaces. Furthermore, SEM systems with diffracted backscattered electron detectors are also used to analyze crystalline-graphic features within many substrates.

The strengths of SEM lie in the study and characterization of solid materials. Although its contributions have been historically dedicated to geological applications, these applications are a minis-cule subset of the many scientific and industrial applications which exist for leveraging this instrumentation. Another strength is that SEM is fairly simple to operate and has a highly intuitive operator interface. A large number of applications will only require minimal preparation of sample surfaces. Also, the acquisition of data is very quick, requiring less than six minutes to generate digital imagery which is easily transferable.

There are a number of limitations to be cognizant of before com-mitting to SEM. As mentioned before, samples must be solid and they need to fit within the microscope chamber. The maximum dimension in sample size in the horizontal direction is usually 10 cm, while in the vertical direction the dimension is typically more lim-ited, rarely surpassing 40 mm. Most SEM instrumentation requires that samples are in a stable state within a vacuum, most typically in the pressure range of 10"5-10"6torr. Sample substrates which are unsuitable for examination under SEM include those materials which will outgas at low pressures, .moisture-laden such as organic


materials, and substrates which can decrepitate. Special EDS X-ray detectors on SEM technology also cannot detect very light elements such as hydrogen, helium and lithium, as well as elements with atomic numbers less than 11. These detectors also have relatively poor energy sensitivity to those elements which are present in low volumes when compared to what are known as "wavelength dis-persive X-ray detectors" (WDS) on most electron microanalyzers. Also, conductive coatings must be applied to insulative samples for study using conventional SEM, assuming the SEM instrument is not capable of operating in a low vacuum mode.

When preparing sample substrates for SEM analysis, a sample must be chosen which will fit into the SEM chamber. Prevention of electrical charge build-up must also be accommodated for elec-trically insulative samples. In these cases, insulative samples are coated with a thin layer of conductive material such as carbon, gold or a range of alloys. The type of coating selected is determined by the type of data to be collected. For example, carbon is the desired coating base if elemental analysis is required. Metal-based coatings are necessary for high resolution electron imaging. And as men-tioned above, insulative samples will not require a conductive coat-ing if the instrument is capable of low-pressure vacuum operation.

It is well known that the wettability of a polymer surface is, in part, determined by surface morphology, and particularly surface roughness. SEM analysis is routinely used to examine the changes in surface morphology created by RF plasma treatment. Enhancements in surface roughness is seen when plasma treatment occurs with argon/oxygen gas mixtures, for example, where other gas combi-nations may not create significant changes in surface roughness. This is an indication that a measured decrease in contact angle may be more dependent upon the polar component within surface-free energy rather than surface roughness. As plasma treatment dwell time increases, there has been empirical evidence of nonhomoge-nous surface etching.

4.6 Transition Electron Microscopy (TEM)

A related technique to SEM surface analysis is Transition Electron Microscopy (TEM). TEM is a scientific instrument that uses electrons rather than light to analyze objects at extremely high resolution.


As such, TEM allows for magnifications of up to 100,000 x with reso-lutions in the nanometer range. TEM has heretofore been primarily used in metallurgy and the biological sciences. The disadvantages associated with TEM primarily lie with the requirements to: 1) slice substrate samples very thinly to assure that there is electron trans-parency, and 2) to place the sample in a vacuum. There are there-fore a number of concerns which surround the use of TEM, such as whether the sample will be as pure as expected, and whether the electron bombardment may damage the substrate. The main surface features which can be observed by TEM are surface rough-ening, the thickness of sputtered metal layers on films, and the presence of any surface defects of hydrogen bubbles.

4.7 Visual Methodologies

In the context of broader commercial and industrial application environments, visual methods of observing changes by plasma surface modification are more prevalent. These include the use of dyne solutions, contact angle and peel force adhesion. The capi-tal investments in these technologies are comparatively low, and they can deliver near-term cursory and qualifying results. We will review each of these techniques in sequence and recommend the most appropriate protocol for maximizing their contribution to sur-face analysis.

4.7.1 Dyne Solutions

Dyne solutions are typically formulated with multiple liquid and pigment components. Ethylene glycol monoethyl ether (HOCH2CH2OC2H5) and formamide (HCONH2) are commonly used in these formulations. There are also nontoxic variations of these solutions encapsulated within felt-tipped dyne pens. Dyne solutions are a primary methodology for determining the sur-face energy of plastic films and other nonporous substrates. This method aligns with the ASTM Standard D2578. The solutions used to formulate a dyne liquid determine the wetting characteristics of the subject substrate. Generally, the ability of a substrate to adhere inks, coatings, or adhesives is directly related to its surface energy. If the substrate surface energy does not exceed the surface tension of the fluid (ink, coating, adhesive) which is intended to overlay it,


Figure 4.3 Dyne solutions applied to untreated and plasma-treated soda lime glass for photovoltaics.

the wetting of the fluid will be impeded and an ineffective bond will result. Therefore, most solvent-based fluids applied to nonpo-rous materials such as plastics need to be plasma treated to 36 to 40 dynes/cm to achieve a bonding state. Water-based fluids usu-ally require a minimum substrate surface energy of 46 dynes/cm to affect adhesion. And, there are some lamination and coating applications which use high-solids fluids which require substrate surface energies of 50 dynes/cm or more. It is therefore mandatory that surface energy of the substrate and the applied fluid must be reviewed before printing, coating, or laminating is attempted.

The dyne solution technique is transferable to many materials. It is critical that the dyne test fluid does not alter the surface properties of the substrate. For example, if the test fluid permeates a porous substrate such as paper and causes swelling, surface energy results will indicate unrealistically easy wetting. Just as non-qualifying, a chemical reaction between the test fluid and the substrate invali-dates results altogether. To ensure test replicability, material prepa-ration and test technique must be standardized. ASTM Standard D618 documents the suggested conditioning methods. However, this standard is untenable for treated film testing, since the mate-rial conditioning times range from 24-96 hours. These condition-ing times may be of value for research and development purposes, but for normal quality control testing, much shorter conditioning times are commonly used. In this vein of thought, standardization of ambient, substrate, and test solution temperatures is critical, as is the inspection methodology. It is recommended that one trainer be identified to instruct all surface energy testers to minimize mea-surement variability. Also, relative humidity should not be exces-sive since higher relative humidity will increase data variability. Finally, the elapsed time between extrusion or coating to surface energy test (or from this test to printing, etc.) must be controlled.


Figure 4.4 ASTM D 2578 standard for mayer rod drawdown of selected dyne solutions for surface tension testing.

Other advisable precautions with this test procedure include the following:

• Avoid touching or contaminating the surface to be tested, as dirty surfaces lose their wettability.

• Avoid using contaminated test fluid; discard every 6 months.

• Do not retest the same location on a sample. • Store and use all test fluids at room temperature. • Use fresh cotton swab applicators for each test.

The appropriate test method using dyne solutions is as follow:

• Select a test sample without contaminating it. For extruded film, one entire web cross-section is recommended.

• Place the sample on a level surface and maintain its orientation so traceability of left vs. right is possible. Anchor the edges to avoid deformation during testing.

• Record ambient temperature and relative humidity. If sample temperature differs from ambient, allow it to stabilize.

• A minimum of three sample points should be tested. • Choose a dyne level test fluid which is slightly lower

than that expected of the test material. • Wet the tip of a new swab with test fluid to coat one

square inch of the test sample.


• The ability of a surface to "wet-out" is an indirect indi-cation of the number of reactive sites available for

• bonding, hence acceptance of ink or adhesive. • Use light application pressure to spread the solution

evenly over this one square inch of the test sample. • Time how long it takes for the applied solution to bead

up. This assessment should be based on the reaction of the fluid in the center of the liquid film.

• If beading does not occur within two seconds, select the next highest level test fluid.

• If beading occurs in less than two seconds, select the test fluid below this level by two to four dynes/cm for test.

• Use a new swab. Never reuse a swab, even in the same dyne level fluid.

• Repeat these steps until you determine the level which comes closest to wetting the surface for exactly two seconds.

• Record your individual test results by location on the sample.

• Calculate the average and range of wetting levels. If the results do not fall within control limits, or if they differ significantly from historic values, it is suggested that two more samples be chosen, and the procedure repeated.

• Typically, multi-operator replicability is usually within about 1.0 dyne/cm.

A variation of this method is outlined by TAPPI T400 "Sampling and Accepting a Single Lot of Paper, Paperboard, Containerboard, or Related Product" and is conducted as follows:

• Select a test sample without contaminating it. • Obtain five specimens equally spaced across a rep-

resentative full width sampling of the web of stock under consideration.

• Prevent the surface of the test specimen from being handled or contaminated in any way.

• Cut the test specimens to a size of 216 mm x 279 mm (8.5" x 11") in the machine direction on a cutting board and label accordingly on a corner which will not affect test results.

•


• Condition the test specimens in accordance with TAPPI T402 "Standard Conditioning and Testing Atmospheres for Paper, Board, Pulp Handsheets, and Related Products."

• Place the test specimen on the clipboard, taking care that it is horizontal.

• Select three dropper bottles of dyne solutions that bracket the anticipated surface treatment level.

• Typically, the upper and lower dyne solutions will be ±2 dynes/cm from the anticipated substrate value.

• Place three drops of the target dyne solution in the center of the specimen, and three drops of the lower and higher solutions 51mm (2 in.) respectively to the left and right of center.

• With a smooth and continuous movement, draw a #6 Mayer rod across the surface of the specimen from top to bottom so as to draw down a thin continuous layer of each of the three dyne solutions over an area of 115 sq. cm. (17.5 sq. in.).

• When the surface treatment level range is narrowed, reduce the interval between surface tension solutions to one unit.

• Retest until it is possible to select the wetting tension mixture that comes nearest to wetting the surface of the film or coating for exactly two seconds.

Aging of the substrate will affect surface characteristics and there-fore dyne level testing. If the constraints of your process preclude good standardization of test timing, designed experimentation should be used to measure the effect of aging on your substrates. Substrate suppliers will typically set material specifications to con-servative levels to compensate for treatment loss. Although surface energy is critically important to many converting operations, the topography of the substrate, coating rheology, and chemical incom-patibility are just as critical. Other important factors include the type of resin used for film or coating, the particular ink or adhe-sive to be used, surface roughness, and the interaction between the media and the reactive sites. By systematically measuring substrate surface energy, a starting point is established from which to resolve adhesion problems.


4.7.2 Contact Angle

There are several contact angle techniques which are applicable for plasma-modified surfaces, principally the Wilhelmy Plate method and goniometry.

The Wilhelmy Plate technique is used to measure the contact angle if the surface tension of the interfacial liquid is known. If the contact angle for a given solid-liquid combination is known, the surface tension of the liquid can be obtained with this method as well. The Wilhelmy plate method consists of a rectangular plate on which an angle is measured. A reservoir of the fluid is kept below the plate. To measure the contact angle, the fluid is elevated towards the plate until it touches the plate. A change in the weight of the plate (AW) occurs because of the liquid adherence to the plate. This change in weight is measured and, with information on the wetted perimeter (p), the contact angle (Θ) is measured from the equation σ cos(6) = AWp. It is important to note that this method of measuring the contact angle is not suitable for rough and porous substrates such as paper. The fibrous surface of paper makes it dif-fcult to measure the perimeter and may also result in wicking of the fluid into the paper, which will result in incorrect weight measure-ments and produce incorrect contact angle results. A modification of the Wilhelmy Plate method is the Single Fiber Wilhelmy method in which the plate is replaced by a single fiber of the substrate (from paper, for example). The single fiber, however, will not be an accu-rate representation of the actual substrate surface.

With goniometry, an image of a water drop is obtained and a contact angle is measured from the drop image. A simple yet uni-versally applied method is to draw a tangent at the solid-liquid interface along the profile of the drop and measure the contact angle. By this method, the obtained angle is highly dependent upon the perception and judgement of the operator. Hence, this method-ology is not suitable for scientific applications.

Contact angle measurements which fit a curve to the water drop edge can eliminate the drop size constraints imposed by several methods. Multiple points on the water drop edge are selected from the images and a curve is fitted to these profile points. Another approach is to model the drops using what is known as the Laplace-Young equation. The numerical solution to this equation was first developed by Bashforth and Adams in 1883. Hartland and Hartley solved the Laplace-Young equation numerically in 1976


and obtained the precise drop profile for different drop parame-ters. Fourteen years later, Cheng followed a similar approach and developed a technique called Axisymmetric Drop Shape Analysis (ADSA) to fit the obtained theoretical water drop profiles to the drop edge obtained from the projected images. One footnote is that Young's equation is applicable to substrates with smooth and uni-form surfaces only. On rough surfaces such as paper, contact angles obtained using Young's correlation would provide incorrect data.

For estimating the contact angle on rough surfaces, a water drop can be used but it will take one of the two forms - a) total wet-ting whereby the liquid wets the entire rough surface; or b) par-tial wetting whereby vapor is trapped between the water drop and the valleys of the rough surface. With total surface wetting, Wenzel developed the equation cos Θ* = r cos Θ in 1936 to model the apparent contact angle (0W) on rough surfaces. The roughness factor (r) rep-resented the ratio of the actual surface area and the projected surface area. For non-wetting surfaces (Θ > 90°), an increase in roughness would increase the contact angle and for wetting surfaces (Θ < 90°), increased surface roughness would in turn reduce the contact angle. In 1944, Cassie and Baxter developed the equation cos Θ* = r ff θγ + f - 1 to model a water drop contact angle on a uniform, heterogene-ous surface where "f" represents the fraction area of each surface under the liquid and θγ represents the contact angle for the same surface [26]. With partial wetting whereby vapor is trapped betw-een the solid substrate surface and the water drop, the equation changes where "i" is the fractional area for the solid-drop interface and "f2" is the fractional area for the porous areas:

cos(GC) = Xfi cos(GYi) 5cos(6C) = f, cos(GY) - f2

Neither the Wenzel equation nor the Cassie-Baxter equation con-sider the irregularities that is evident at the solid-drop-vapor con-tact line.

The accuracy in contact angle measurement is highly dependent upon the type of image processing technique that is applied for estimating the water drop edge from the drop images. Typically, the drop image is converted into a gray-scale image. The gray-scale image will have a dark foreground representing the water drop and a white background. The edge of the drop, however, will not be accurate since it consists of step changes in the profile. With


an accurate drop profile, the edges are expected to be smooth and continuous.

With contact angle, the accuracy of edge detection is critical to defining surface tension of a substrate. The levels of uncertainty that exists within water drop edge detection causes the contact angles obtained to be a function of the methodology employed for edge detection. Said differently, the accuracy as to how the solid-liquid interface is detected plays a very important part in determining an accurate contact angle. With some methodologies, a Laplacian smoothing curve (referencing Laplace-Young equation mentioned above) is employed to assist in determining an accurate contact angle. The length-weighted Laplacian smoothing approach calcu-lates an average drop edge length to weight the magnitude of the allowed node movement. The approach is highly sensitive to ele-ment edge lengths and tends to average these lengths to form better shaped drops [27]. Once the fit between the Laplacian curve and the drop profile is accomplished, the coordinates of the substrate's surface are used to sever the Laplacian curve and the angle is calcu-lated. For smooth substrate surfaces, surface reflection provides an improved detection of the substrate-liquid drop interface. A rough and porous surface makes detection of this interface quite difficult. Extremely high contact angles further challenge the ability to detect the precise substrate-liquid drop interface. It has been observed that smaller contact angle drops will result in a larger error as opposed to larger drops at a constant contact angle, and that contact angles of < approximately 30° and >150° are highly dependent on the accu-racy of interface detection methodology.

When measured accurately, static contact angles can provide dependable information about the interfacial tensions between a solid substrate and a liquid drop. However, advancing and reced-ing contact angles (dynamic contact angles) will provide informa-tion about the dynamic interaction of solid substrate and liquid drop. An advancing contact angle is determined with the use of a syringe pump which injects fluid continuously into the drop on the solid substrate surface at a constant rate. When the liquid drop initially meets the solid substrate, it forms the contact angle. As the syringe pump injects more liquid, the drop will increase in volume, the contact angle will increase, and its boundary will remain sta-tionary until it is suddenly pushed outward. The contact angle of the drop just prior to its outward push is termed the advancing contact angle. A receding contact angle is subsequently measured


by withdrawing or sucking the liquid back out of the drop. As the drop decreases in volume, the contact angle will decrease. But its boundary will remain stationary until it is suddenly pulled inward. The contact angle that the drop exhibited immediately before being pulled inward is termed the receding contact angle. The difference between advancing and receding contact angles is termed "con-tact angle hysteresis" which can be used to characterize surface heterogeneity, roughness, and mobility. This hysteresis occurs as a result of the metastable states which are observed as the liquid drop interfaces with the surface of a solid at the solid /liquid interface. Because there are free energy barriers between these metastable states, a contact angle at equilibrium is nearly impossible to mea-sure. When a perfect substrate surface is wet by a pure liquid, only one stable contact angle is predicted to be determined. In reality, there are virtually no perfect surfaces. Therefore, to fully character-ize any surface, it is important to measure both the advancing and receding contact angles and to report the difference between them as the contact angle hysteresis.

Humidity and temperature play key roles in determining accu-rate contact angles. As temperature increases, the amount of water vapor in the air needed for saturation increases. When ambient air is not humidified, the water drops formed for contact angle mea-surements will evaporate and therefore the obtained contact angle will differ from the actual one. Humidity is routinely increased by placing water filled containers in the system's heat enclosure. To verify the rate of evaporation, a small droplet of water is injected into the chamber at set temperature and the time required for the drop to evaporate is measured. If the evaporation rate is <0.5μ1/ min, the contact angle

measurement can proceed. Sessile drops are created by inject-ing water through a hypodermic needle. Typically, Teflon tubing is used to convey water from the syringe to the needle. The tempera-ture of the enclosure is increased by increasing the voltage to ther-moelectric heaters. Once the substrate plate reaches the prescribed temperature, the enclosure is kept in the same state for up to 30 minutes. This allows the entire chamber to attain an equilibrium temperature, and for the contact angle procedure to be executed with high repeatability.

It should be noted that there are also a wide range of non-water contact angle measurement methodologies available for use. For example, organic solvents such as alkanes and alcohols can be used


for characterization of surface tension. Organic solvents added to the syringe (typically with a stainless steel needle) dispensed on the sample surface will be relatively small, not requiring an exhaust hood when measuring contact angle. Organic solvents have inher-ently low surface tension. Ethyl alcohol has a surface tension of approximately 22.3 m N / m , making it suitable for testing on low surface energy polymer substrates, for example. However, it can be expected that lower contact angle values will be evidenced, and will be attributable to the adsorption (with the hydroxyl group) of the alcohol molecules at the polymer-liquid interfaces. The value of contact angles will depend on the alcohol used, and will be rela-tively independent of the substrate.

4.7.3 Peel Force Adhesion

Peel force adhesion as a surface test method quantifies the secure nature of adhesion, or peelability, of pressure sensitive materials. Peel adhesion is the force required to remove pressure sensitive coated material such as an adhesive tape, which has been applied to a substrate surface under specified conditions and removed from the substrate at a specified angle and speed. A load cell measures the peel force and this is recorded in the data acquisition unit. The results are subsequently processed locally or in an external PC. The sensitivity of the test may be adjusted by changing the load cell. Measurement of peel adhesion at 90° will normally yield a lower value than at 180°. Test pressure sensitive strips should be 25 mm wide and have a minimum length of 175 mm in the machine direc-tion. The average test result will be typically defined in Newtons per 25 mm width.

Peel adhesion force is dwell time dependent. All pressure sen-sitive adhesives applied to test tapes experience a certain degree of deformation after being applied to the test substrate. Once peel

Table 4.1 Peel adhesion comparison of APT vs. chemical primer.

POLYIMIDE STRAIN GAUGE FILM - METAL ADHESION RESULTS

SAMPLE

Standard Wet Chemical Primer Treatment

APT Treatment - He + 0 2 Reactive Gas

PEEL FORCE (g)

9.0

11.0


pressure ceases, these adhesives will continue to deform and result in different adhesion forces at varying time frames. Most solvent and water-based tape adhesives will have a tendency to continue wetting to the substrate surface even though the application pres-sure is no longer present. Upon peeling, the fracture mode can be adhesive failure at the start and becomes cohesive failure after elapsed time. The fracture mode transformation will occur at the time the adhesion force between adhesive and the test substrate is greater than the cohesion of the adhesive itself. The peel adhe-sion will ultimately not change significantly with dwell time for adhesive systems having high cohesion. This is because these adhe-sives are not easily deformed under lightly applied pressure and therefore will not intimately contact the test substrate, most partic-ularly when pressure is no longer being applied. Because substrate cleanliness is so important to gathering reliable data, appropriate measures must be taken to ensure freedom from dirt, grease, and other contaminants. Finally, the adhesive tape/substrate can also be exposed to different conditions to determine the influence on the bond, such as climate ageing (heat, cold, humidity), heat, UV-light and hot water.

atmospheric pressure plasma for surface modification (wolf/atmospheric) || characterization methods...

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