Coating Materials News Vol 10 Issue 3
Choosing Between Thermal Evaporation & Sputter Deposition
As our 10 year anniversary celebration goes on, we continue reviews of popular discussions that have appeared in past issues of CMN. This issue covers two topics: the most common techniques of physical vapor deposition (PVD), and adhesion and defect and stress reduction problems.
The most commonly used PVD evaporation techniques are thermal and sputtering. In thermal evaporation, the source material is brought to evaporation temperature either by the heat generated by the resistance of a metal container or by bombardment of a beam of high energy electrons. Many variations of these basic processes are in use, and the question ‘which technique is best’ frequently arises. Previous issues of CMN have discussed the varied processes and applications of sputtering, and compared them with e-beam deposition. Since this topic has been one of the most popular among CMN readers, we briefly review the comparison in this issue.The most commonly used PVD evaporation techniques are thermal and sputtering. In thermal evaporation, the source material is brought to evaporation temperature either by the heat generated by the resistance of a metal container or by bombardment of a beam of high energy electrons. Many variations of these basic processes are in use, and the question ‘which technique is best’ frequently arises. Previous issues of have discussed the varied processes and applications of sputtering, and compared them with e-beam deposition. Since this topic has been one of the most popular among readers, we briefly review the comparison in this issue.
The salient features that distinguish sputter and thermal deposition are:
Veteran readers of CMN will recall articles appearing in the first two years of publication describing in some depth the technique of sputtering [CMN V1 Issue 4 Oct.-Dec. 1991; V2 Issue 3 Jul.-Sept 1992; and V2 Issue 4 Oct.-Dec.1992]. Ten years later there is more crossover, but thermal evaporation is still predominantly the technique of choice for optical coatings while sputtering finds its best application to metal traces in semiconductor circuits, wear- resistant coatings, transparent conductive layers, thin film resistors, and metallization for food packaging and thermal control. The latter four applications are done on large areas of flexible substrates in roll and web coaters. Low emittance coatings are applied to windows at millions of sq. meters per year for energy conservation, for example.
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The Basics of the Sputter Process
Three predominant configurations of sputter systems were illustrated in CMN V2 Issue 4, 1992. Of these, planar magnetron is most widely used in both in-line and web machines for volume production. Recent variations designed to increase sputter rate include the dual AC magnetron where alternately one target operates in sputter mode while its twin is in surface clean mode. The roles of the targets alternate at up to 40 kH, and higher effective rates are reported.
The sputtering process requires the generation of energetic heavy + ions (generally Argon) that are responsible for sputtering atoms from the target (cathode). An electric field between target and anode (substrate holder) contains the electron cloud. Magnetic fields cause the electrons to spiral and collide with Ar atoms, which are then ionized. The Ar+ are accelerated, strike the target, and knock off target atoms. Many variations to this planar magnetron configuration have been developed for special applications. More detail is given in CMN V6 Issue 3, 1996.
Many coating applications in semiconductor fabrication, windows, lamp reflector production, capacitor, and food packaging (to name a few) require films of pure metals. Sputtering is efficient for these applications. When compound films are required, as in optical, wear resistance and decorative coating applications, the process is modified. The sputter yields of metals is greater than that of compounds therefore compounds are generated during the sputtering of the metal target by introducing a reactive gas along with argon. Oxides, nitrides, carbides, etc. are deposited by reactive sputtering. Sputtering is used to produce many square kilometers of transparent conductive coatings per year for many applications (CMN V10 Issue 2, June 2000). Sputtering sources are easily adapted to web and in-line continuous coating. This is especially true for less critical depositions as, for example, metal layers. Monitoring thicknesses for optical applications is more difficult, however than in a batch coater. Monitoring techniques are improving, however, and the near future will witness a movement toward more optical coatings being deposited in large areas and volumes on web and in-line sputter machines.
The ground-breaking work by Movchan and Demchishin and subsequent researchers, known as the Structure Zone Model (SZM), gives us good insight into the influences of pressure and temperature on the microstructure of film layers (Figure 1) [CMN V2, Issue 3 1992].
The high adatom energy contributed by the sputtered species provides similar if not equivalent influence on the growth structure as substrate temperature. That energy is provided by the bias, either applied or generated, that accelerates sputtered species. Sputtered energies can be 1 to 10 eV, while evaporated energies are <1ev. the="" result="" is="" that="" the="" sputtered="" films="" are="" denser,="" generally="" amorphous="" in="" nature="" and="" more="" adherent="" than="" evaporated="" films.="" additional="" topics="" involving="" sputtering="" appeared="" in="">CMN V9, Issue 2 1999, V8 issue 2, 1998, and V4 Issue 4, 1993.
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Problems of Adhesion, Stress and Defects in Thin Films
Adhesion, stress and defects are ongoing problems whose relative magnitudes vary with material, substrate, deposition technology, and application. Interestingly, adhesion quality, stress level and defect density are interrelated with nature; film layers possessing high stress often exhibit adhesion loss and defects. We see through our studies that the microstructure of the deposited film is important in relation to film strength and adhesion [CMN V1, Issue 3, 1991]. The type of microstructure determines the intrinsic stress of the layer according to the SZM. Amorphous materials are stronger and show greater adherence than granular or columnar loose structures because the amorphous growth indicates more complete nucleation on the substrate surface.
The deposition technique is also important to avoid defects. For example, a source that spatters particulates, even microscopic, will encourage the growth of nodules that include the particle. These are weak points that can fail with abrasive contact or thermal cycling. Some evaporated materials or their prepared forms produce more particulate emission than others. CERAC attempts to prepare materials in physical forms or chemical state to minimize spatter and outgassing. Some materials simply deposit with high stress, and require careful consideration of surface preparation, perhaps including the deliberate deposition of a nucleation layer of a different material. Other materials, particularly fluoride compounds, benefit from the introduction of "doping" materials; example: CERAC IRX and IRB materials [CMN V3 Issue 3, 1993 and V5 Issue 1, Jan-Mar. 1994].
Substrate surface cleaning techniques were discussed in an issue devoted to adhesion, CMN V6 Issue 1, 1996. Mechanical, chemical, and ion abrasion are mechanisms used to various degrees and on specific substrate compositions.
It is generally held that sputtered films adhere better than evaporated films, but just how that works is subject to controversy. It is believed that the mechanism is that energetic adatoms, Ar+ and electrons scour the substrate surface of contaminants and perhaps create metastable states or microscopic surface damage that initiate nucleation. It is not solely the energy of the adatoms that increases adhesion since that energy is generally <10 ev,="" comparable="" with="" the="" bonding="" energy="" of="" some="" common="">
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Our understanding and technology of thin film deposition and materials has improved greatly in the past decade, but work has not stopped. We can expect greater advancements in the future. It is CERAC’s goal that materials research and improvements keep pace with deposition technology advances.
*Newsletters beginning with Vol. 6 are available from the CMN archives page of this web site. Contact CERAC for printed copies of referenced newsletters prior to Vol. 6.
If you have a question or a topic you would like us to consider for a future issue of CMN, e-mail your requests to email@example.com or fax them to 414-289-9804.
(S.F. Pellicori is available for private consulting on matters concerning optical thin films. Please contact him directly for more information)
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