Tailoring Polymer Properties with Ion Beams

Nathan Capps, Dan Carter, Greg Roche, Advanced Energy Industries Inc., Fort Collins, Colo. -- Semiconductor International, 7/1/2000

Polymers have evolved in the past century to become seminal materials in a wide variety of industrial, electronic and research applications. Due to the incredible range of characteristics displayed by polymeric materials, their potential applications appear unlimited. This unique class of compounds finds uses in products as varied as contact lenses, artificial organs, structural materials, food packaging and now as potential substrates for high-density, flexible circuit boards. The capability of polymers to act as low-k dielectric materials makes them increasingly attractive to a variety of semiconductor and circuit fabrication applications. The versatility of these materials stems from the vast array of mechanical properties, surface properties and chemical compositions available in commercial polymers. However, despite their merits, polymers are not without problems for certain applications.

Many polymer applications are limited by poor adhesion between a deposited material and the polymer surface. In any application where adhesion is critical, differences in chemical and physical surface characteristics between substrate and deposited film must be addressed. One application directly facing these challenges is the use of polymers as circuit boards with copper interconnect components. Increasing demand for smaller devices continues to drive circuit designs into the vertical dimension and raises concerns about electrical properties of both interconnect and insulating (substrate) materials. These demands spawn technologies aimed at achieving higher integration densities and meeting the novel challenges associated with multilevel metallization (MLM) schemes. Copper is attracting much attention as a possible solution to some material requirements. It is able to handle the high current densities necessitated by smaller device features and is a more suitable interconnect metal than aluminum. The ability to deposit quality copper films is currently the focus of much research, and numerous methods such as chemical vapor deposition (CVD) and plasma-assisted chemical vapor deposition (PA-CVD), among others, are being evaluated to optimize process characteristics and film quality.

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1. Ion beams could improve adhesion on polymers by surface roughening or interfacial mixing.

Concurrent with the need for good interconnect materials is the need for robust, versatile and adherent dielectric materials that are easily fabricated on a production scale. For some time, the use of many polymers as substrates has been hindered by poor adhesion of the metal/polymer interface. While there is some flexibility in manipulating characteristics of the metallic films, the ease in manipulating the surface characteristics of the polymer, combined with the variety of techniques available to achieve this, suggests the logical focus is toward tailoring the surface of the polymer to improve adhesion of the metal.

In addition to metal/polymer adhesion issues, examples abound of applications that would benefit directly from surface modification processes of polymers. In many biomedical applications, the mechanical behavior of a certain polymer may be very desirable, but its surface properties may render it incompatible with biological systems. A reliable surface modification process could make available entirely new applications for polymers and bring into feasibility numerous concepts previously fettered by adhesion and interface difficulties. These examples illustrate the need for a technology capable of achieving reliable, controllable surface modification of polymers.

Gridless ion sources offer a remarkable parameter space for surface treatments, including ion energies, fluxes and feed gas chemistries. Altering the physical and/or chemical surface properties could help enhance metallization, adhesion and other applications. This article offers a brief overview of this enabling technology and how it can address surface modification strategies beneficial to adhesion and polymer metalization.

Surface modification methods

Much research has been directed in recent years to investigate the surface modification of polymers. Different methods are being studied, ranging from traditional (wet-chemical) processes to plasma treatments to novel fabrication schemes, and there are benefits and drawbacks to each. Despite the enormous body of work on this topic, the search continues for a universal solution. The ideal surface treatment process would possess several characteristics. It should be specifically localized to the surface and not significantly affect the bulk properties of the polymer. Mechanical strength and thermal characteristics could then coexist with specifically tailored surface properties. The treatment technology should be able to introduce a wide range of physical and/or chemical changes to the surface, including surface texturing, microstructure formation, chemical functional group implantation, crosslinking or functional group abstraction. The technology should be capable of inducing these changes in short treatment times, enabling high throughput fabrication. Finally, the treatment should be flexible and accommodate the wide range of polymers likely to benefit by its use. This entails addressing the issues of substrate heating, contamination control and the efficiency of processes likely to scale up to production levels.

Ion beam treatments offer significant promise in meeting these requirements. In contrast to UV bombardment, which can cross-link polymer chains to significant depths and possibly alter bulk properties, ion beam treatments affect structures via a molecular and not radiative vehicle, thus confining their effect within certain atomic penetration and diffusion boundaries. In addition, there is an enormous selection of ion beam treatments capable of altering chemical and physical surface properties of polymers. Because of the wide application of plasma and ion beam treatments to semiconductors in recent years, many of these techniques have evolved into production-scale solutions. Therefore, the means already exist for integrating this technology into production applications for a variety of materials. Finally, ion beam treatments could be easily and effectively combined with traditional wet-chemical processes with capabilities of sophisticated chemical surface modification.

Table 1. Polymer Surface Modification Techniques

Treatment technique	Method	Applications
Ultraviolet bombardment (UV lamps)	Energetic photons cause cross-linking and free radical formation in polymers.	Limited utility for extensive surface modification. Alteration of bulk properties of polymer. Requires additional (2nd step) treatments.
RF plasma	Bombardment of polymer surfaces with energetic neutrals, ions, radicals, photons and electrons.	Surface modification achievable without altering bulk polymer properties. Non-directional nature affects throughput and treatment characteristics. Variety of surface modifications available.
Gridless ion sources	Bombardment of polymer surfaces with highly directional ions of variable energy. Selectable surface chemistries based on choice of process gas.	Localized surface modification. Directional nature, energetic species could give high process efficiencies. Wide range of surface modifications available. Easily combined with deposition techniques.

Several plasma-based techniques have the capability for polymer surface modification, each with unique merits that justify its application to a particular process. Some of the most common methods are rf plasma treatment, corona discharges and ion beams. Table 1 illustrates some of the potential capabilities of these techniques. Of these methods, ion beam sources offer some of the most promising and attractive capabilities for adhesion improvement processes. Ion beams are capable of highly anisotropic etching because of the ion beam directionality. This characteristic could be applicable to precision etch applications, such as microelectric machine (MEM) fabrication. Ion beam technology has been proven to be scalable in high-volume industrial applications; it is commonly used in such applications as glass cleaning and implanting for tool and die materials. Associated with ion beam technology is a wide parameter space for adjustable ion energies, a critical factor in successful polymer treatment. Finally, broad-beam ion sources, with their ability to operate in reactive gases, offer the possibility of chemical as well as physical modification of a polymer surface.

Ion beams could improve adhesion on polymer surface by various mechanisms. Surface roughening (Fig. 1b) is one of the principal ways to improve adhesion of a coating, and has been a successful application of ion beam technology on various substrates and films, such as DLC and Al₂O₃ films to a variety of glass and ceramic substrates. Another technique beneficial to adhesion is interfacial mixing (Fig. 1c). In this process, energetic ions bombard a surface during a deposition process and cause mixing of substrate and coating on the atomic level. While commonly associated with very high ion energies used for inorganic interfaces, this technique also could be well suited to a polymer adhesion problem, since lower ion energies could produce interfacial mixing between polymer substrates and a metal coating. This use of broad-beam ion source technology for removal of a sharp boundary layer and the replacement with a small gradient layer holds great promise. Ion beams also can be used to introduce chemical functional groups on the surface that can lead to a greater affinity for a variety of materials, such as traditional adhesives, ceramics or metallic oxides. Finally, ion beams could be used in the formation of extended gradient interfaces between a substrate and a coating. This strategy represents perhaps the ultimate in adhesion improvement, with the total elimination of a distinct interface.

Gridless ion sources

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2. In a gridless ion source, ions are accelerated by the space charge field that runs perpendicular to the magnetic field. Since ions originate at different points along this space charge, the resulting ion beam has a broad ion energy distribution.

Gridless ion sources (Fig. 2) are a unique technology capable of remarkable process variety and have many potential applications to polymer surface modification. These sources are an elegant solution to ion beam production and are rapidly increasing in popularity as new utilities for this technology are recognized. A magnetic field is formed in front of an anode; this restricts electron motion toward the anode and forms a space charge field. Electron collisions ionize feed gases dispersed through or about the anode. Ions originating within the plasma region are are repelled from the anode and accelerated by the space charge field that runs perpendicular to the magnetic field. Since ions originate at different points along this space charge, and therefore experience a different magnitude of repulsion, the resulting ion beam has a broad ion energy distribution.

Gridless ion sources often are referred to as "Hall-current" or "closed-drift" ion beam sources. Electrons confined within the magnetic field experience a Hall-effect force running parallel to the anode face, similar to a magnetron sputtering source. The anode is usually a ring, circular channel, cone or cylinder that allows the electrons to drift continuously around a closed-path plasma region.

Table 2. Gridless Ion Source Process Options

Process gas	Result	Benefit
Argon (Ar)	Polymer surface roughening Surface cross-linking Microstructure formation Surface cleaning of contaminants/residual organics	Improved adhesion through physical means Improved adhesion through clean surface More stable polymer surface
Oxygen (O2), Nitrogen (N2)	Some surface roughening Introduction of chemical functionality to surface O2 surface cleaning of residual organic contaminant material or release agents	Improved adhesion Changes in polymer surface wettability Possible adhesion improvement by altered surface chemistry
Hydrocarbons (CXHY) Butane (C4H10) Ethylene (C2H4)	Deposition of adherent carbon-based films DLC, Amorphous polymeric films Combined with O2, N2 to tailor chemical composition	Possible gradient interface layers between polymer/coating Protective overcoats Adhesion improvement
Ion-beam assisted processes Ion bombardment during deposition process Argon (Ar) Reactive Gases (O2, N2, H2)	Localized energetic bombardment Increased adatom mobility Bombardment with activated species during reactive deposition	Stress relief in sputtered films (Cu) Better adhesion Stability of a reactive deposition process Fabrication of gradient interfaces Film densification

Gridless ion sources offer significant benefits to numerous processes and process environments. These devices produce ion beams with high current densities and with a broad distribution of energies as well as a wide range of operation. Gridless sources can operate in vacuum pressures from 0.01 to 100 mTorr. This ability to operate at magnetron sputtering pressures allows their use in a variety of deposition processes. Sputter deposition of copper could be enhanced by the use of gridless ion sources, for example, operating with either reactive or nonreactive gases. Ion beams from gridless ion sources consist of a broad distribution of ion energies. These energies can range from nearly zero to the magnitude of the anode potential, with the highest number density centered about half the anode potential. This broad energy spectrum provides a gridless ion source with a unique level of flexibility to participate in cleaning, surface modification and deposition processes. The high current densities provided by this technology are attractive from an efficiency standpoint Table 2 lists various process options using gridless ion sources.

Polymer surface treatments

3. Untreated polytetrafluoroethylene (PTFE) surface.

To illustrate the versatile nature of ion beam treatment of polymers, several experiments have been performed on a variety of substrates. As mentioned above, polymer surface roughening can be achieved with gridless ion source technology. Figure 3 is a scanning electron micrograph (SEM) image of the surface of untreated polytetrafluoroethylene (PTFE). While there are some surface features, this topology is typical of an untreated polymer surface.

4. PTFE surface after ion beam treatment.

Figure 4 shows a SEM image of PTFE surface after treatment with an argon/oxygen ion beam from an Advanced Energy Linear Ion Source. The applied voltage was 3000 eV at 2.2 mTorr chamber pressure. This image clearly illustrates the changes in surface topography attainable with gridless ion source technology. Figure 5 is a SEM image of a cross-section of PTFE after treatment with a 2700 eV (applied voltage) Ar ion beam, again from an Advanced Energy Linear Ion Source. This image illustrates the localized nature of the surface modification, as features are visible on the surface yet the internal bulk polymer remains relatively smooth.

5. SEM cross-section image of PTFE surface after ion beam treatment.

The previous example illustrates the capability of gridless ion sources to physically modify the polymer surfaces. As mentioned earlier, it is well known that roughening of surfaces will improve adhesion of a deposited film, and preliminary results suggest the adhesion of copper on acrylobutastyrene (ABS) polymer can be improved with ion beam treatment. Approximately 1 µm of copper was sputtered onto samples of ABS plastic that had been treated with an Ar ion beam to achieve surface roughening. Samples that had undergone ion beam pre-treatment showed no delamination and good adhesion when subjected to a tape test. Control samples performed in the same pumpdown cycles showed poor adhesion to the substrate and delaminated under a tape test. These results suggest the adhesion of copper onto polymeric materials can be greatly enhanced by the use of ion beam pre-treatment.

6. ABS surface after ion beam treatment.

Figure 6 and Figure 7 show the ABS polymer surface after and before ion beam treatment, respectively. The treatment was an Ar ion beam at an applied voltage of 1500 eV for 3 minutes. Some small amount of surface roughening is detectable at these magnifications, and it is possible that further evidence of roughening may be visible with higher resolutions. Figure 8 is an image of a cross-section regime of ABS/polymer interface. The sputtered copper film is visible on top of the polymeric layer, and some evidence of surface roughening is visible. These images, coupled with the data for adhesion of these materials, suggest ion beam treatment could provide enhanced adhesion characteristics through physical surface modification.

Surface modification mechanisms

7. Untreated ABS polymer surface.

Ion beam treatments can alter the adhesion characteristics of a deposited film and a polymer substrate in various ways. Polymer chains can cross-link as a result of energetic ion bombardment, producing a much more rigid area at the surface and maintaining an induced surface roughness. This is beneficial, as a cross-linked region will have greater mechanical stability and therefore is less likely to suffer from thermal effects of a subsequent sputter deposition. Cross-linking of a polymer surface also is useful in maintaining the benefits of functional group implantation, as attached surface functional groups (O, N) are less able to migrate into the bulk polymer. In addition to cross-linking, ion beam treatment can generate free radical sites on a polymer surface. This makes available a range of graft copolymerization strategies, in which the treated surface is exposed to an organic monomer or inorganic complex capable of free radical polymerization. These compounds can be chosen to produce a very specific chemistry on a polymer surface. Finally, as mentioned before, interfacial mixing of a polymer substrate and a deposited film offers an attractive solution to adhesion problems. Because of the "soft" nature of polymeric materials, this process could be achieved with ion energies in the range of 1-5 KeV, making gridless ion beam technology an excellent choice for this strategy. These capabilities of gridless ion sources make them an attractive technology, which could be used in conjunction with traditional approaches such as spin-coated, low-k dielectrics as well as novel metallization and deposition processes.

8. Cross-section image of sputtered copper/polymer interface.

Conclusion

The wide variety of treatments available offer exciting new areas for research and development to conclusively identify and optimize both the method and effect of ion beam treatment. Gridless ion beam ion technology offers a wide parameter space for exploration, along with a field-proven reputation of reliable operation in fabrication environments, and likely may offer a viable solution to copper/polymer adhesions and other applications. •

Nathan E. Capps earned his M.S. in chemistry at Colorado State University and his B.S. in chemistry at Florida State University. He is an application development engineer working on film process development in the Applications R&D Group at Advanced Energy Industries Inc.

Dan Carter is applications manager at Advanced Energy Industries Inc. He holds B.S. and M.S. degrees in material science and engineering from Purdue and Northwestern Universities, respectively. Prior to joining Advanced Energy, he worked for 13 years in semiconductor fabrication. At Advanced Energy, his primary focus is on applications of ion beam and ICP sources.

Gregory A. Roche is director of the Applications R&D Group at Advanced Energy Industries Inc. He holds a B.S. in electrical engineering from the University of California, Irvine. Before joining Advanced Energy, he worked for 20 years in semiconductor processing and semiconductor process equipment design. His current work focuses on design and application of novel plasma sources.

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