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Chapter 13 Coating techniques
437
PART 3: CHEMICAL SURFACE COATING
Chapter 13
Coating techniques
Coating techniques can be defined as all procedures which share the final aim to create a thin layer on a foreign substrate. The thickness of such a coating varies from a monomolecular layer (nm) to several millimetres. The concept of coating is very extended. Some books on coating techniques describe the art of painting. In this book, the discussion will be restricted to procedures, developed to create a stable surface coating on an inorganic substrate, by means of deposition techniques or by means of a series of chemical reactions of the substrate with various reagents. The coating techniques will be classified as Chemical Vapour Deposition (CVD), Physical Vapour Deposition (PVD), Atomic Layer Epitaxy (ALE) and Chemical Surface Coating (CSC).
1 Chemical Vapour Deposition (CVD) ~ The CVD process involves the reaction of a mixture of gases with a heated substrate. Due to the thermoshock at the substrate's interface, the gases decompose and solid decomposition products deposit at the surface. During the last decades, several alternative CVD methods have been developed, allowing the creation of a coating at lower temperatures and/or in a more localized way. Some of these alternative techniques will be discussed.
438
1.1 Conventional Chemical Vapour Deposition Conventional Chemical Vapour Deposition (also called Thermal Chemical Vapour Deposition) is the simplest CVD technique, in which a mixture of reactive gases (typically metalhalides and hydrides) are decomposed by and deposited on a heated substrate ~'2. At the moment, it is the only CVD technique that has been implemented on a commercial basis to produce large amounts of cheap coated materials. A typical configuration of a thermal CVD reactor is shown in figure 13.1. The applications of conventional CVD are far too extended to cover in this introduction. Tables 13.1 and 13.2 present the typical process parameters and the typical deposition reactions in a CVD experiment. The range of these parameters indicates the scope and versatility of CVD. The high process - temperature (1100- 2300 K) causes a strong diffusion of the coating into the substrate, resulting in a good attachment. The CVD technique does not suffer from the 'line-of-sight' effect (cfr. PVD): the substrate is coated entirely with a relatively uniform layer. At the same time, this high temperature is the biggest disadvantage of the CVD technique. The different thermal expansion coefficients of substrate and coating cause thermal cracks in the ceramic coating upon cooling. This is the main reason for the development of a number of alternative CVD techniques, allowing a significant lowering of the deposition temperature.
439
71
6
To ve,nt
ooooo
9
3
!
II 13
! /
Figure 13.1 Schematic diagram showing various components of a CVD system (1: reactor," 2: heating element; 3: reaction tube; 4: water-cooled end flanges; 5: power controller; 6."pressure indicator," 7." temperature sensor; 8,10,11: precursor gas tanks, 9." metal halide (liquid) vaporizer," 12: particulate trap," 13: gas scrubber," 14: flow meter; 15: flow meter valves; 16: gas tank regulators; 17." substrate support; 18: substrates).
440 Table 13.1 Typical process-parameters of a conventional CVD experiment Temperature Pressure Precursors
1100 K - 2300 K < 0.1 - 101300 Pa reactive gases (metal halides) reducing gases (HE) inert gases (Ar, N~) other gases (CH4, CO2, NH3)
Table 13.2 Typical deposition reactions, occurring in a conventional CVD experiment Pyrolysis Reduction Oxidation Hydrolysis Co-reduction
CH3SiC13 --, SiC + 3HC1 WF6 + 3H2--, W + 6HF Sill4 + 02 ~ SiO2 + 2H2 2A1C13 + 3H20 ~ A1203 + 6HC1 TIC14 + 2BC13 + 5H2---, TiB2 + 10HC1
1.2 Metal-organic CVD (MOCVD) This technique uses organometallic compounds with a relatively low decomposition temperature ( < 1073 K) as coating precursors. There is a lot of interest for MOCVD, especially by manufacturers of semi-conductors, who already employ it commercially for the deposition of semi-conducting films of GaAs, InAs, InP and ZnSe. The films are very thin, in the/~ngstrom range, and are usually epitaxial. Amorphous as well as crystalline films can be deposited, but the latter usually have higher defect densities, limiting their usefulness in critical applications. In microelectronics, this technique is referred to as organo-metallic vapour phase epitaxy (OMVPE). A detailed account of this technique is given by Dapkus. 3 The MOCVD technique has also been used to deposit a number of refractory compounds for a wide range of applications. One of the advantages of this technique is the lower deposition temperature, which makes it very suitable for deposition on substrates, which are thermally sensitive, such as steels. Wear - resistant tungsten carbide coatings have been deposited on steel, using WF 6 and suitable hydrocarbon gases at temperatures below 873 K. 4'5
441 1.3 Plasma-Enhanced CVD (PECVD) Plasma-enhanced chemical vapour deposition has gained importance rapidly in recent years, because this technique provides some unique advantages over conventional CVD. The important advantages include lower deposition temperatures, deposition of non-epuilibrium phases and a better control of stoichiometry and purity of deposits. In this technique, the activation energy for the breakdown of reactive species, and their subsequent interaction with other species to form a deposit, is provided by the high kinetic energy of electrons in the plasma (figure 13.2). I
I RF plate
(cathode)
(~
RF generator (or DC power source)
~"//////:~ ates
IiiiIII-'l
Source gas
Pump
Figure 13.2 Schematic representation of a radial flow plasma CVD reactor.
The plasma is created by an electrical field between both parallel plates (see figure), ionizing the gas volume inbetween. In a plasma, the energy is transferred by collisions between all particles. Due to their smaller mass, the energy of the electrons increases much faster than the energy of the heavier ions. This means that mainly the electrons are responsible for the ionization processes and the formation of reactive free radicals. The big energy difference between electrons and ions is reflected in the respective temperatures" the electron temperature of a typical plasma is about
442 10000 K, whereas the gas temperature usually does not exceed 850 K, provided that the pressure is low ( < 1300 Pa). The dissociation of gas molecules in a plasma involves the formation of intermediate, highly reactive fragments, which are very unstable under normal circumstances. Therefore, PECVD allows the deposition of quite unique materials with very unusual properties. One of such unique coatings is Diamond Like Carbon (DLC). The conventional synthesis of synthetic diamonds requires extremely high temperatures and pressures. By PECVD, Diamond Like Carbon is created under mild conditions by the decomposition of methane in H2/CH 4 mixture. The applications of DLC are numerous" coatings for cutting tools, optical fibres, electronic devices for reading magnetic tapes, or even protective coatings in chemical reactors.
1.4 Laser CVD (LCVD) This technique has also been used more and more in recent years, primarily in the microelectronics field. The activation of gaseous species, in this case, is achieved by shining a laser beam in the reactor. Even though both with plasma and laser techniques the same general result is achieved, there are some significant differences between the two techniques. The main difference is the ability of the laser to create high energy electrons in a very narrow energy band as compared to the electron energy distribution in a typical plasma. Again, as a result of such a localized activation of the gas volume, the deposition temperatures in LCVD can be considerably lower than in the conventional CVD. Basically, there are two types of laser CVD techniques, as illustrated in figure 13.3. In a photolytic or photochemical LCVD experiment, the gas absorbs the laser energy whereas the substrate is transparent. The gas molecules are ionized and fragmentation occurs, resulting in the deposition of a coating on the substrate, which is at relatively low temperature. In this technique, the wavelength of the laser can be chosen in such a way as to allow only selected gaseous species to be activated, permitting the formation and deposition of selective film compositions.
443
Volatile Products Reactant ~ ~ ~ ,,. - - "" ~ ~ ~ k
r_JAltemate !-I Substrate -]Position
t__
. _ , , ~ hv _ ~ ~ ~
[-- Substrate I/L~ ~Hot
Spot
~_~ Deposit
Substrate ~ Products
Deposit
Reactant
Figure 13.3 Schematic arrangement of Laser CVD systems. Left: photolytic LCVD; right." pyrolytic LCVD. In a pyrolytic or thermal LCVD experiment, the gas is transparent and the substrate absorbs the laser energy. This creates a so - called hot- spot on which a normal thermal CVD process occurs. Pyrolytic LCVD allows a very precise localization of the coating. In a sense, this technique may be compared to the 'cold - wall' CVD technique in which the substrate may be heated by passing an electric current through it (resistance heating), or by induction, where the substrate itself acts as a susceptor. In these cases, the gas volume is not heated significantly (hence the name 'cold - wall' CVD). The main difference between the cold-wall CVD and the pyrolytic laser CVD is that in the latter, the heated area can be localized and scanned very precisely.
1.5 Fluidized-bed CVD (FB CVD) This is a relatively special technique which combines the principles of fluidized - bed heating and CVD. It is primarily used to coat powders of very fine size with suitable films for special applications. The most prominent application of this technique is in the coating of nuclear fuel particles used in high-temperature gas-cooled reactors (HTGR). A typical fluidized-bed CVD reactor is shown schematically in figure 13.4. Considerable work is this area has been carried out at the Oak Ridge National Laboratory where coatings of high and low density graphite and of SiC have been deposited on uranium oxide and thorium oxide microspheres. 6 The purpose of these
444
SIG-4T PORT COOt.e~, wATER
I----COOUNG WATER OUTLET
EXHAUST /~;).l'_~ED_~ E Ds
GRAPHITE CHAt~IE3ER GqAP~TE I~ATfJG ELEIvqENI CARBON Ir[LT I~SULAT~Oq
CL._VEW e.OFL[
WATER-COOL[~ SIEEL JACKEl
OUTER _J JACKET
ELECTRiCAl INSULAIOR
I I | ,'-ce~..:
II t~SULAT~
WATER-C00LED COPPER ELECIROC~
COOLING1 L INNER JACKET WATER
.,i,tn-
*
COOLED GAS IN&CTOR f
ii i' (;AS ~NLET
!t
d~
Figure 13.4 Schematic representative of a fluidized bed CVD reactor.
films is to contain the fission products of the nuclear reaction in order to minimize exposure and contamination from radio- active species. The inner, low density coatings protects the outer coatings from fission - recoil damage and provides a free volume for the fission products. The outer, high - density coating acts as a pressure vessel and diffusion barrier for solid fission fragments. Another prominent application of FBCVD is in the manufacture of high purity silicon. Silicon seed particles are fluidized in a bed, in which a mixture of silane and hydrogen is introduced. Decomposition of silane on the silicon particles results in the formation of a pure silicon film. It is of importance to control the process in order to minimize homogeneous nucleation of silicon dust by appropriately designing gas distribution system and controlling process parameters. 7,s
445
1.6 Chemical Vapour Infiltration (CVI) The increasing use of light ceramic composites for high temperature and space applications has stimulated the development and optimization of the Chemical Vapour Infiltration technique. The use of conventional ceramic techniques for the fabrication of fibre-reinforced composites damages the fibres both mechanically as chemically. Also, the high process temperature causes a thermal degradation of the fibres. The substrate in a CVI experiment is a highly porous material. This porous structure is infiltrated by vapours at considerably lower temperatures and pressures (compared to a conventional CVD experiment), causing a deposition on and between the fibres. This yields a very strong composite with high density. Three possible reaction schemes can be discerned: * isothermal diffusion with a concentration gradient * diffusion with a thermal gradient * diffusion with a pressure gradient Figure 13.5 shows a schematic representation of the second possibility. 9 The main advantage of this configuration is that the reaction only occurs on the hot surface, away from the gasinlet. As the deposition proceeds, the density and thermal conductivity of the coated zone increases, lowering the hot reaction zone towards the gasinlet. In this way, the entire volume is infiltrated uniformly and progressively. CVI has already been successfully used for the fabrication of fibre-reinforced composites of A1, A1N, BN, SiC, TaC and TiB2 .7'8'9'1~
1.7 Materials and applications The types of materials deposited by CVD, range from pure metals to compounds, ceramics, powders, whiskers and composite coatings. The various types of metals and compounds which have been successfully deposited by CVD are shown in table 13.3. In most of these cases, the precursors are metal halides which are unstable in the temperature range of deposition. When suitable halides cannot be obtained, metalorganic compounds can be used.
446
Heating element
Hot zo~ 1473 K
~Retainin~
Exhaust gas
Inf'fltratcd composite N "////d "//'/,~
;'///2 "///// ~.//// z/'/.,~
N
Water-cooling holder
Fibrous preform
Coating gas
Figure 13.5 Schematic representation of chemical vapour infiltration process. Table 13.3 Materials deposited by CVD Metals: Compounds: Ceramics:
A1, As, Be, Bi, Co, Cr, Cu, Fe, Ge, Hf, Ir, Mo, Nb, Ni, Os, Pb, Pt, Re, Rh, Ru, Sb, Si, Sn, Ta, Th, Ti, U, V, W, Zr. Also carbon and boron. II-VI and III-V compounds, borides, carbides, nitrides and silicides of transition metals, as well as sulphides, phosphides, aluminides, etc. A1203, A1N, B203, BN, SiC, Si3N4, UO2, Y203, ZrOz, etc.
Table 13.4 shows some typical applications of chemical vapour deposition. This table illustrates the wide variety of applications and the versatility of the CVD technique.
447
Table 13.4 Typical applications of chemical vapour deposition * Tribological coatings * Wear-resistant coatings * High-temperature coatings for oxidation resistance * Dielectric/insulating films * Optical/reflective films * Photovoltaic films
* Decorative films * Superconducting films * Emissive coatings * Coatings for fibre composites * Free-standing structural shapes * Powders and whiskers
By far the largest areas of application of CVD include manufacture of powders (for pigments), micro - electronics (involving dielectric films, optical films, super conducting and emissive films), and tribology. Irrespective of the application of the CVD coating, one of the most important criteria in the selection of the coating process is a consideration of the substrate/gas interaction. In conventional CVD, the reaction temperatures are typically above 1073 K. Thus, those substrates which undergo phase transformation related dimensional changes at high temperature become unsuitable. Many metals also show a propensity for reaction with furnace gases at these temperatures, limiting their use as substrates. The presence of residual stresses at the interface, or in the coating, can be a cause of serious concern in some applications, while degradation of the interfacial region below the coating/substrate interface can often cause failure of the coated component due to mechanical shock before the advantages of the coating are manifested. Thus, the most suitable substrates are refractory metals (such as tungsten, molybdenum), certain ceramics (such as alumina, mullite, silicon nitride), and graphite. High - speed steels and some other high - alloy steels have been used in selected applications, but the use of steels, in generally is limited in conventional CVD. Cemented tungsten carbide is used as a substrate for depositing wear-resistant coatings of TiC and TiN for applications in metal - cutting. Graphite and molybdenum are often substrates of choice for fabricating free - standing parts by using them as shaped mandrels. Of course, in many cases, the choice of substrate is limited by the application. In such cases, a proper design of the coating chemistry is important to ensure overall enhancement of properties with a minimum compromise of substrate properties. ~
448 Coatings used for tribological applications require good adhesion, a low coefficient of friction, high hardness and wear resistance. Coatings of TiC, TiN, CrTC3 and tungsten carbide have been used in many of these applications such as forming tools, ball beating components, machine parts, gears, steel cutting tools, surgical and prosthetic implements, etc. Titanium nitride is a popular coating for decorative applications in addition to its excellent frictional characteristics. In many tribological applications, environmental degradation is also a factor. Therefore, these coatings are also required to provide improved chemical resistance. Hard coatings have been applied to many types of forming tools, such as deep-drawing punches and dies, wire - drawing dies, injection moulding dies, etc. It was shown that the application of coating resulted in an eight - fold increase in service life at 80% reduction in tool costs. Another well-known example of improvement in service life by the applications of a wear-resistant coating is that of cemented carbide cutting tool inserts. These tools are typically coated with TiC, TiN, A1203, TaC, HfN, etc. These coatings impart improved abrasion resistance, chemical resistance, frictional characteristics by deflecting the heat generated during metal cutting into the metal chips, away from the tool, thereby preventing softening of the tool due to excessive heat at the tool tip. Coatings for high temperature service require good thermal and chemical stability. Refractory compounds such as various oxides, silicides and aluminides are commonly used in these .applications. Other coatings which are also useful are SiC, Si3N4 and certain refractory metals such as iridium. Many high temperature applications involve particulate erosion ablation, corrosive environments and severe thermal cycling. Coatings in such applications are expected to withstand these conditions. Another situation involves coatings used in fusion reactor components, where erosion of wall coatings can lead to poisoning of the plasma. In such cases, resistance to sputtering erosion, and reaction with hydrogen ions is important. Thermal barrier coatings for superalloy components in high temperature turbines and engines require good adhesion, a careful match of thermal expansion coefficients through multiple layers, and good resistance to oxidation, erosion and corrosion. The CVD technique can be successfully used in depositing such multiple layers with excellent control of thickness and uniformity.
449
2 Physical Vapour Deposition (PVD) Physical Vapour Deposition (PVD) is another coating technique. The reactants (precursors) are solids, which are forced in a gaseous state. This can be done by simple heating, but mostly, this procedure involves ion - bombing in order to create a plasma. The gaseous phase deposits on the solid substrate at relatively low temperatures. The main advantages of the PVD technique are: * the purity of the coating; * the broad range of suitable precursors; * the low temperature of the substrate; * the fine structure of the coating, which makes polishing unnecessary. The main disadvantages are: * the low deposition rate; * the complexity of the process and the equipment; * the high demands concerning the purity and cleanliness of the substrate' surface; * the line - o f - sight effects. The line-of-sight effect is the biggest disadvantage. It means that the part of the substrate, which is located directly above the source, will achieve a much thicker coating that other parts of the substrate. As a consequence, only very simple geometric forms can be coated uniformly by PVD, provided that either the substrate is rotated, or that multiple sources are present in strategic places in the reactor. PVD is a time consuming process" not only the deposition rate is relatively low, also severe precautions are necessary concerning the cleanliness of the substrate. This involves long pretreatment procedures. Depending on the way the solid precursor is volatized, three PVD methods are discerned" * thermal PVD * sputtering * ion-plating
450
2.1 Thermal Physical Vapour Deposition P VD Figure 13.6 shows the basic configuration of a thermal PVD reactor.
[
]S
\
Iv lip
Figure 13.6 Schematic representation of a thermal PVD reactor (S= substrate," V= source P= pumps).
Since in a thermal PVD experiment, the solid source material is volatized by simple heating, the choice of precursors is rather limited (melting temperature < 1850 K). Conventional thermal PVD is mostly used for the deposition of metals. PVD of a ceramic layer involves the introduction of an additional gas. However, the energy of the gas is much too low to form a ceramic reaction product. For example, the formation of TiN from Ti and N2 needs much higher temperatures than the normal PVD process temperatures. This supplemental activation of the gas mixture is established by placing a positive electrode in the gas volume. This technique is called Activated Reactive Evaporation (ARE) and is illustrated in figure 13.7.
451
II +
Is
I
TE
\
~
/
V
II
P
Figure 13.7 Activated reactive evaporation (G= gas supply," S= substrate; V= source; TE= positive electrode," P= pumps).
2.2 Sputtering If a solid or liquid at any temperature is subjected to bombardment by suitable high energy atomic particles (usually ions), it is possible for individual atoms to acquire enough energy via collision processes to escape from the surface. This means of causing ejection of atoms from the surface is called sputtering. The atoms ejected from the surface can be used in depositing a coating on a substrate (figure 13.8).
I I C
I
I
IT
Is lip
Figure 13.8 Sputtering (G= gasinlet," T= target," S= substrate," P= pumps).
452 In the magnetron-sputtering technique (figure 13.9), the target is subjected to a magnetic field. This causes an additional ionization of the plasma near the target, allowing lower target temperatures.
II
~
L--
M
I
!r
i
is il
P
Figure 13.9 Magnetron sputtering (G= gasinlet," M= magnetron; T= target; S= substrate; P= pumps).
2.3 Ion plating Ion plating is a process in which a substrate is subjected to ion bombardment both before and during the time it is being coated. The coating process used in ion plating is vacuum evaporation from resistance - heated sources. The vacuum procedures in ion plating are the same as those used in sputtering in that the system is first pumped to a good vacuum and argon gas is then admitted to establish a state of dynamic epuilibrium in the chamber. The chamber pressure remains constant with argon being pumped out of the chamber at the same rate as it is flowing into the chamber. Chamber pressures in the order of 5 Pa are fairly standard in ion plating. A typical ion plating system is depicted in figure 13.10. Radio-frequency power is applied to the substrate, as in sputter cleaning, for the purposes of generating a plasma and of causing ion bombardment on the substrate surface. After sputter-cleaning has removed the contaminants from the substrate to establish atomically clean surfaces, the evaporation source is activated to begin the coating process.
453
II a
§+
T
-I
I r +
T
IS II P
Figure 13.10 Ion plating (G= gasinlet; S= substrate," T= target," P= pumps). 3 Atomic Layer Epitaxy Chemical Vapour Deposition involves a very complex mixture of gases and/or ions, causing numerous uncontrollable side reactions. From a chemical point of view, these reactions are extremely difficult to monitor. A basic understanding of all reactions occurring in a CVD experiment is lacking at the present time. This was recknognized by Suntola ~ in the late 1970's, when he developed a new coating technique: Atomic Layer Epitaxy (ALE). Atomic layer epitaxy is a method for producing thin films and layers of single crystals one atomic layer at a time, utilizing a self-control obtained through saturating surface reactions. ALE is thus based on separate surface reactions between the growing surface and each of the components of the compound, one at a time. These components are supplied in the vapour phase, either as elemental vapours or as volatile compounds of the elements. ALE was originally developed to meet the needs of improved ZnS thin films and dielectric thin films for electroluminescent thin film display devices. As an illustration of the ALE process, figure 13.11 summarizes the basic sequences of ALE in two alternative ways for producing ZnS.
454
ooooo, 9 OO,o.
'I ~ CI + Surplus H2S [
Zn (a)
2Zn+S
Figure 13.11 the reactants.
-. 2ZnS
, I (b)
ZnCI + H S -. ZnS + 2HCI
ALE process for ZnS. (a) Zn and S as the reactants; (b) ZnCI2 and H:S as
In each reaction there are sites for only one monolayer to make a bond with the original surface. A basic condition for a successful ALE process is that the binding energy of a monolayer chemisorbed on a surface is higher than the binding energy of subsequent layers on top of the formed monolayer. The temperature of the substrate is used as the primary controlling parameter. It is adjusted low enough to keep the monolayer on the surface until reaction with the following reactants takes place, but high enough to re - evaporate any of the subsequent layers on the top of the monolayer. The control of the monolayer can further be influenced with the aid of a laser beam or other extra energy. The greater the difference between the bond energy of a monolayer and the bond energies of the subsequent layers, the better the self- controlling characteristics of the process. Several general reaction mechanisms can be discerned in the ALE process.
The
simplest reaction sequence is an elemental reaction, which can be represented as reaction (A).
B(s) + A(g) --, BA(s)
(A)
455 The reaction of elemental Zn and S to form a ZnS layer is a typical example of such reaction. However, there might be different reasons for using compounds of the elements A and B as the reactants, rather than the elemental vapours of A and B. The most important reason is that for many metals, the vapour pressure is (too) low, which makes it very difficult to prevent condensation of the element at low substrate temperatures. In this cases, compound exchange reactions are used:
B(s) + AX(g) --, B-AX(s) B-AX(s) + B Y ( g ) - , B-AB(s) + XY(g)
(B)
The reaction ZnC12 + H2S --* ZnS + 2HC1 is a typical example. In many cases, the compound AX is a metal halide and the compound BY is a hydride. However, also organometallic compounds are attractive alternatives as reactants in an ALE process, owing to their high vapour pressure. There is a large amount of information available on many important organometallic compounds, because of the wide use in the CVD technology. Originated in the late 1970's, ALE has become a widely used synthesis route for thin epitaxial layers. Many ALE coated materials are produced commercially today. A very extended review on the achievements of the ALE technique, written by the pioneer in the field, is found in reference 11.
4 Molecular Layering In Eastern Europe, parallel to the development of ALE, a quite similar coating technique was developed, called Molecular Layering. 12'13'~4'~5 The principles of Molecular Layering are based on the irreversible chemical reactions of the substrate with a functional reagent. In this way, monolayers can be created to be used as such or to undergo further reaction with a second reagent. Its principles are visualized in figure 13.12.
4~
a) layer of given thickness
pB
functionalgroups BB
BB
BB
~
/
B B B B B
~
~
\
+Ac2 + BC-
BB
solid body b) layers with given disposition of monolayers of different chemical nature /
+AC 4
c
cc
BB
\
/
BB
\
/
BB
k
/
C
B
CC CC C "(~ "(~/ "(1~/
cc, ,c + I~4
v
- BC ~
+
-BC c) muldcoml~nent monolayers with given C CC CC CC C
monolayer
mixture
C CC CC CC C \
I
\_.J
\_.../ \
I
+ (xAC4 + yNC 4 ) - BC, - zAC4
C~(~/C
B CN'C B
+ yNC,
-B---F-"
C CCCC C ~A)/ ~ ~
- ~, \ \' ~~\ ~~\ \'~~\ \ . ~ .~~ %
+ zMC4
BB
~"
- BC
"
Figure 13.12 Principles of the molecular layering technique, according to Malygin; taken from ref. (22) with permission.
457 An important application of this technique is the creation of thin transition metal oxides on a substrate, by reacting consecutively with a metalchloride (TIC14) or oxychloride (CRO2C12, VOC13) and water, according to reaction (C). 16'17'~g ( - Si-OH),, + TiCl 4
(~- Si-O).TiCl,.
+
~
(4-n)H20
( - - Si-O)nTi(OH)4_n
+
( - Si-O).TiCl4..
~
+
(-= Si-O).Ti(OH),.
TiC! 4
n HC!
+
(4-n)HCl
---> ( - - Si-O).(Ti.O)mTiCl4_ m
(~- Si-O).(Ti-O)mTiCl4_ m +
(4-m)H20
-,,
....
(C)
Another typical application is the creation of sensors. Vanadium modified silica, according to reaction (D), yields a very useful compound for the visual control of humidity in gas media. 19 (-- SiOH). + VOCI 3 ~
(---Si-O).VOCI3_ n + nHC!
(D)
The different colour changes, compared to commercial cobalt containing silica, are presented in table 13.5.
Table 13.5 Comparison of the sensitivity (colour changes) of V-modified silica (ML-method) and commercial Co-containing silica Relative humidity % Dew point K
0.5 233
1.6-4.6 6-10 243-253 255-260
10-13 261-263
15-45 264-279
48-95 281-292
colour V-silica of the external layer commerc, Co-silica
lemon
bright yellow
light orange
dark orange
from red brown to dark
blue
blue
light blue
light rosy
rosy
458
5 Chemical Surface Coating In the early 90's, a new technique, 'Chemical Surface Coating (CSC)' was developed by Vansant, Gillis-D'Hamers, Van Der Voort and Vrancken, in order to create thin ceramic layers on a substrate by successive chemical modifications. The principles and developments of this technique are the subject of a separate section of this book (cfr. chapter 14).
References
D.G. Bhat, A review of chemical vapour deposition-techniques, materials and applications; surface modification technologies, eds. T.S. Sudarshan and D.G. Bhat, 1988.
.
D
C.F. Powell, in Vapour Deposition, C.F. Powell, J.H. Oxley and J.M. Blocher jr. eds., John Wiley and Sons, New York, 1966, pp. 249-276.
3.
D.P. Dapkus, Ann. Rev. Mater. Sci., 1982, 12, 243.
4.
R.A. Holtz, R.E. Benander and R.D. Davis, U.S. Patent 4 427 445, 1984.
5.
N.J. Archer and K.K. Yee, Wear, 1978, 48, 237.
6.
R.B. Pratt et al., Nucl. Appl., 1969, 6, 241.
0
S. Lai, M.P. Dudokovic and P.A. Ramachandran, Chem. Eng. Sci., 1986, 41, 633.
8.
G.C. Hsu et al., NASA Tech. Brief., 1985, 9, 104.
9.
D.P. Stinton, A.J. Caputo and R.A. Lowden, Ceramic Bulletin, 1986, 65, 347.
10.
C. Hollabaugh, in Proceedings of sixth international conference on chemical vapour deposition, The Electrochemical Society Inc., Princeton, New Jersey, 1977, pp. 419-429.
11.
T. Suntola, Materials Science Reports, 1989, 4, 261 - 321 and references therein.
12.
A.A. Chuiko, Teoret. i. Eksperim. Khimiya, 1987, 5, 597.
13.
V.B. Aleskovskiy, Stehiometriya i sinteztverdyh soedinery, L/Nauka, 1976.
459 15.
S.I. Koltsov and V.B. Aleskovskiy, Zhurnal Prilk. Khimii, 1967, 40, 207.
16.
M.N. Tzetkova, I.M. Yur'evskaya and A.A. Malygin, Zhurn. Prikl. Khimii., 1982, 2, 256.
17.
S.D. Dubrovenskiy, A.E. Emelyanov and A.V. Zimin, Zhurn. Prikl. Khimii, 1992,
65, 2259. 18.
E.A. Avrutina, Avtoreferat diss. kandidat khim. nauka., Leningrad, 1989.
19.
A.A. Malygin, private communication.
PART 3: CHEMICAL SURFACE COATING
Chapter 13
Coating techniques
Coating techniques can be defined as all procedures which share the final aim to create a thin layer on a foreign substrate. The thickness of such a coating varies from a monomolecular layer (nm) to several millimetres. The concept of coating is very extended. Some books on coating techniques describe the art of painting. In this book, the discussion will be restricted to procedures, developed to create a stable surface coating on an inorganic substrate, by means of deposition techniques or by means of a series of chemical reactions of the substrate with various reagents. The coating techniques will be classified as Chemical Vapour Deposition (CVD), Physical Vapour Deposition (PVD), Atomic Layer Epitaxy (ALE) and Chemical Surface Coating (CSC).
1 Chemical Vapour Deposition (CVD) ~ The CVD process involves the reaction of a mixture of gases with a heated substrate. Due to the thermoshock at the substrate's interface, the gases decompose and solid decomposition products deposit at the surface. During the last decades, several alternative CVD methods have been developed, allowing the creation of a coating at lower temperatures and/or in a more localized way. Some of these alternative techniques will be discussed.
438
1.1 Conventional Chemical Vapour Deposition Conventional Chemical Vapour Deposition (also called Thermal Chemical Vapour Deposition) is the simplest CVD technique, in which a mixture of reactive gases (typically metalhalides and hydrides) are decomposed by and deposited on a heated substrate ~'2. At the moment, it is the only CVD technique that has been implemented on a commercial basis to produce large amounts of cheap coated materials. A typical configuration of a thermal CVD reactor is shown in figure 13.1. The applications of conventional CVD are far too extended to cover in this introduction. Tables 13.1 and 13.2 present the typical process parameters and the typical deposition reactions in a CVD experiment. The range of these parameters indicates the scope and versatility of CVD. The high process - temperature (1100- 2300 K) causes a strong diffusion of the coating into the substrate, resulting in a good attachment. The CVD technique does not suffer from the 'line-of-sight' effect (cfr. PVD): the substrate is coated entirely with a relatively uniform layer. At the same time, this high temperature is the biggest disadvantage of the CVD technique. The different thermal expansion coefficients of substrate and coating cause thermal cracks in the ceramic coating upon cooling. This is the main reason for the development of a number of alternative CVD techniques, allowing a significant lowering of the deposition temperature.
439
71
6
To ve,nt
ooooo
9
3
!
II 13
! /
Figure 13.1 Schematic diagram showing various components of a CVD system (1: reactor," 2: heating element; 3: reaction tube; 4: water-cooled end flanges; 5: power controller; 6."pressure indicator," 7." temperature sensor; 8,10,11: precursor gas tanks, 9." metal halide (liquid) vaporizer," 12: particulate trap," 13: gas scrubber," 14: flow meter; 15: flow meter valves; 16: gas tank regulators; 17." substrate support; 18: substrates).
440 Table 13.1 Typical process-parameters of a conventional CVD experiment Temperature Pressure Precursors
1100 K - 2300 K < 0.1 - 101300 Pa reactive gases (metal halides) reducing gases (HE) inert gases (Ar, N~) other gases (CH4, CO2, NH3)
Table 13.2 Typical deposition reactions, occurring in a conventional CVD experiment Pyrolysis Reduction Oxidation Hydrolysis Co-reduction
CH3SiC13 --, SiC + 3HC1 WF6 + 3H2--, W + 6HF Sill4 + 02 ~ SiO2 + 2H2 2A1C13 + 3H20 ~ A1203 + 6HC1 TIC14 + 2BC13 + 5H2---, TiB2 + 10HC1
1.2 Metal-organic CVD (MOCVD) This technique uses organometallic compounds with a relatively low decomposition temperature ( < 1073 K) as coating precursors. There is a lot of interest for MOCVD, especially by manufacturers of semi-conductors, who already employ it commercially for the deposition of semi-conducting films of GaAs, InAs, InP and ZnSe. The films are very thin, in the/~ngstrom range, and are usually epitaxial. Amorphous as well as crystalline films can be deposited, but the latter usually have higher defect densities, limiting their usefulness in critical applications. In microelectronics, this technique is referred to as organo-metallic vapour phase epitaxy (OMVPE). A detailed account of this technique is given by Dapkus. 3 The MOCVD technique has also been used to deposit a number of refractory compounds for a wide range of applications. One of the advantages of this technique is the lower deposition temperature, which makes it very suitable for deposition on substrates, which are thermally sensitive, such as steels. Wear - resistant tungsten carbide coatings have been deposited on steel, using WF 6 and suitable hydrocarbon gases at temperatures below 873 K. 4'5
441 1.3 Plasma-Enhanced CVD (PECVD) Plasma-enhanced chemical vapour deposition has gained importance rapidly in recent years, because this technique provides some unique advantages over conventional CVD. The important advantages include lower deposition temperatures, deposition of non-epuilibrium phases and a better control of stoichiometry and purity of deposits. In this technique, the activation energy for the breakdown of reactive species, and their subsequent interaction with other species to form a deposit, is provided by the high kinetic energy of electrons in the plasma (figure 13.2). I
I RF plate
(cathode)
(~
RF generator (or DC power source)
~"//////:~ ates
IiiiIII-'l
Source gas
Pump
Figure 13.2 Schematic representation of a radial flow plasma CVD reactor.
The plasma is created by an electrical field between both parallel plates (see figure), ionizing the gas volume inbetween. In a plasma, the energy is transferred by collisions between all particles. Due to their smaller mass, the energy of the electrons increases much faster than the energy of the heavier ions. This means that mainly the electrons are responsible for the ionization processes and the formation of reactive free radicals. The big energy difference between electrons and ions is reflected in the respective temperatures" the electron temperature of a typical plasma is about
442 10000 K, whereas the gas temperature usually does not exceed 850 K, provided that the pressure is low ( < 1300 Pa). The dissociation of gas molecules in a plasma involves the formation of intermediate, highly reactive fragments, which are very unstable under normal circumstances. Therefore, PECVD allows the deposition of quite unique materials with very unusual properties. One of such unique coatings is Diamond Like Carbon (DLC). The conventional synthesis of synthetic diamonds requires extremely high temperatures and pressures. By PECVD, Diamond Like Carbon is created under mild conditions by the decomposition of methane in H2/CH 4 mixture. The applications of DLC are numerous" coatings for cutting tools, optical fibres, electronic devices for reading magnetic tapes, or even protective coatings in chemical reactors.
1.4 Laser CVD (LCVD) This technique has also been used more and more in recent years, primarily in the microelectronics field. The activation of gaseous species, in this case, is achieved by shining a laser beam in the reactor. Even though both with plasma and laser techniques the same general result is achieved, there are some significant differences between the two techniques. The main difference is the ability of the laser to create high energy electrons in a very narrow energy band as compared to the electron energy distribution in a typical plasma. Again, as a result of such a localized activation of the gas volume, the deposition temperatures in LCVD can be considerably lower than in the conventional CVD. Basically, there are two types of laser CVD techniques, as illustrated in figure 13.3. In a photolytic or photochemical LCVD experiment, the gas absorbs the laser energy whereas the substrate is transparent. The gas molecules are ionized and fragmentation occurs, resulting in the deposition of a coating on the substrate, which is at relatively low temperature. In this technique, the wavelength of the laser can be chosen in such a way as to allow only selected gaseous species to be activated, permitting the formation and deposition of selective film compositions.
443
Volatile Products Reactant ~ ~ ~ ,,. - - "" ~ ~ ~ k
r_JAltemate !-I Substrate -]Position
t__
. _ , , ~ hv _ ~ ~ ~
[-- Substrate I/L~ ~Hot
Spot
~_~ Deposit
Substrate ~ Products
Deposit
Reactant
Figure 13.3 Schematic arrangement of Laser CVD systems. Left: photolytic LCVD; right." pyrolytic LCVD. In a pyrolytic or thermal LCVD experiment, the gas is transparent and the substrate absorbs the laser energy. This creates a so - called hot- spot on which a normal thermal CVD process occurs. Pyrolytic LCVD allows a very precise localization of the coating. In a sense, this technique may be compared to the 'cold - wall' CVD technique in which the substrate may be heated by passing an electric current through it (resistance heating), or by induction, where the substrate itself acts as a susceptor. In these cases, the gas volume is not heated significantly (hence the name 'cold - wall' CVD). The main difference between the cold-wall CVD and the pyrolytic laser CVD is that in the latter, the heated area can be localized and scanned very precisely.
1.5 Fluidized-bed CVD (FB CVD) This is a relatively special technique which combines the principles of fluidized - bed heating and CVD. It is primarily used to coat powders of very fine size with suitable films for special applications. The most prominent application of this technique is in the coating of nuclear fuel particles used in high-temperature gas-cooled reactors (HTGR). A typical fluidized-bed CVD reactor is shown schematically in figure 13.4. Considerable work is this area has been carried out at the Oak Ridge National Laboratory where coatings of high and low density graphite and of SiC have been deposited on uranium oxide and thorium oxide microspheres. 6 The purpose of these
444
SIG-4T PORT COOt.e~, wATER
I----COOUNG WATER OUTLET
EXHAUST /~;).l'_~ED_~ E Ds
GRAPHITE CHAt~IE3ER GqAP~TE I~ATfJG ELEIvqENI CARBON Ir[LT I~SULAT~Oq
CL._VEW e.OFL[
WATER-COOL[~ SIEEL JACKEl
OUTER _J JACKET
ELECTRiCAl INSULAIOR
I I | ,'-ce~..:
II t~SULAT~
WATER-C00LED COPPER ELECIROC~
COOLING1 L INNER JACKET WATER
.,i,tn-
*
COOLED GAS IN&CTOR f
ii i' (;AS ~NLET
!t
d~
Figure 13.4 Schematic representative of a fluidized bed CVD reactor.
films is to contain the fission products of the nuclear reaction in order to minimize exposure and contamination from radio- active species. The inner, low density coatings protects the outer coatings from fission - recoil damage and provides a free volume for the fission products. The outer, high - density coating acts as a pressure vessel and diffusion barrier for solid fission fragments. Another prominent application of FBCVD is in the manufacture of high purity silicon. Silicon seed particles are fluidized in a bed, in which a mixture of silane and hydrogen is introduced. Decomposition of silane on the silicon particles results in the formation of a pure silicon film. It is of importance to control the process in order to minimize homogeneous nucleation of silicon dust by appropriately designing gas distribution system and controlling process parameters. 7,s
445
1.6 Chemical Vapour Infiltration (CVI) The increasing use of light ceramic composites for high temperature and space applications has stimulated the development and optimization of the Chemical Vapour Infiltration technique. The use of conventional ceramic techniques for the fabrication of fibre-reinforced composites damages the fibres both mechanically as chemically. Also, the high process temperature causes a thermal degradation of the fibres. The substrate in a CVI experiment is a highly porous material. This porous structure is infiltrated by vapours at considerably lower temperatures and pressures (compared to a conventional CVD experiment), causing a deposition on and between the fibres. This yields a very strong composite with high density. Three possible reaction schemes can be discerned: * isothermal diffusion with a concentration gradient * diffusion with a thermal gradient * diffusion with a pressure gradient Figure 13.5 shows a schematic representation of the second possibility. 9 The main advantage of this configuration is that the reaction only occurs on the hot surface, away from the gasinlet. As the deposition proceeds, the density and thermal conductivity of the coated zone increases, lowering the hot reaction zone towards the gasinlet. In this way, the entire volume is infiltrated uniformly and progressively. CVI has already been successfully used for the fabrication of fibre-reinforced composites of A1, A1N, BN, SiC, TaC and TiB2 .7'8'9'1~
1.7 Materials and applications The types of materials deposited by CVD, range from pure metals to compounds, ceramics, powders, whiskers and composite coatings. The various types of metals and compounds which have been successfully deposited by CVD are shown in table 13.3. In most of these cases, the precursors are metal halides which are unstable in the temperature range of deposition. When suitable halides cannot be obtained, metalorganic compounds can be used.
446
Heating element
Hot zo~ 1473 K
~Retainin~
Exhaust gas
Inf'fltratcd composite N "////d "//'/,~
;'///2 "///// ~.//// z/'/.,~
N
Water-cooling holder
Fibrous preform
Coating gas
Figure 13.5 Schematic representation of chemical vapour infiltration process. Table 13.3 Materials deposited by CVD Metals: Compounds: Ceramics:
A1, As, Be, Bi, Co, Cr, Cu, Fe, Ge, Hf, Ir, Mo, Nb, Ni, Os, Pb, Pt, Re, Rh, Ru, Sb, Si, Sn, Ta, Th, Ti, U, V, W, Zr. Also carbon and boron. II-VI and III-V compounds, borides, carbides, nitrides and silicides of transition metals, as well as sulphides, phosphides, aluminides, etc. A1203, A1N, B203, BN, SiC, Si3N4, UO2, Y203, ZrOz, etc.
Table 13.4 shows some typical applications of chemical vapour deposition. This table illustrates the wide variety of applications and the versatility of the CVD technique.
447
Table 13.4 Typical applications of chemical vapour deposition * Tribological coatings * Wear-resistant coatings * High-temperature coatings for oxidation resistance * Dielectric/insulating films * Optical/reflective films * Photovoltaic films
* Decorative films * Superconducting films * Emissive coatings * Coatings for fibre composites * Free-standing structural shapes * Powders and whiskers
By far the largest areas of application of CVD include manufacture of powders (for pigments), micro - electronics (involving dielectric films, optical films, super conducting and emissive films), and tribology. Irrespective of the application of the CVD coating, one of the most important criteria in the selection of the coating process is a consideration of the substrate/gas interaction. In conventional CVD, the reaction temperatures are typically above 1073 K. Thus, those substrates which undergo phase transformation related dimensional changes at high temperature become unsuitable. Many metals also show a propensity for reaction with furnace gases at these temperatures, limiting their use as substrates. The presence of residual stresses at the interface, or in the coating, can be a cause of serious concern in some applications, while degradation of the interfacial region below the coating/substrate interface can often cause failure of the coated component due to mechanical shock before the advantages of the coating are manifested. Thus, the most suitable substrates are refractory metals (such as tungsten, molybdenum), certain ceramics (such as alumina, mullite, silicon nitride), and graphite. High - speed steels and some other high - alloy steels have been used in selected applications, but the use of steels, in generally is limited in conventional CVD. Cemented tungsten carbide is used as a substrate for depositing wear-resistant coatings of TiC and TiN for applications in metal - cutting. Graphite and molybdenum are often substrates of choice for fabricating free - standing parts by using them as shaped mandrels. Of course, in many cases, the choice of substrate is limited by the application. In such cases, a proper design of the coating chemistry is important to ensure overall enhancement of properties with a minimum compromise of substrate properties. ~
448 Coatings used for tribological applications require good adhesion, a low coefficient of friction, high hardness and wear resistance. Coatings of TiC, TiN, CrTC3 and tungsten carbide have been used in many of these applications such as forming tools, ball beating components, machine parts, gears, steel cutting tools, surgical and prosthetic implements, etc. Titanium nitride is a popular coating for decorative applications in addition to its excellent frictional characteristics. In many tribological applications, environmental degradation is also a factor. Therefore, these coatings are also required to provide improved chemical resistance. Hard coatings have been applied to many types of forming tools, such as deep-drawing punches and dies, wire - drawing dies, injection moulding dies, etc. It was shown that the application of coating resulted in an eight - fold increase in service life at 80% reduction in tool costs. Another well-known example of improvement in service life by the applications of a wear-resistant coating is that of cemented carbide cutting tool inserts. These tools are typically coated with TiC, TiN, A1203, TaC, HfN, etc. These coatings impart improved abrasion resistance, chemical resistance, frictional characteristics by deflecting the heat generated during metal cutting into the metal chips, away from the tool, thereby preventing softening of the tool due to excessive heat at the tool tip. Coatings for high temperature service require good thermal and chemical stability. Refractory compounds such as various oxides, silicides and aluminides are commonly used in these .applications. Other coatings which are also useful are SiC, Si3N4 and certain refractory metals such as iridium. Many high temperature applications involve particulate erosion ablation, corrosive environments and severe thermal cycling. Coatings in such applications are expected to withstand these conditions. Another situation involves coatings used in fusion reactor components, where erosion of wall coatings can lead to poisoning of the plasma. In such cases, resistance to sputtering erosion, and reaction with hydrogen ions is important. Thermal barrier coatings for superalloy components in high temperature turbines and engines require good adhesion, a careful match of thermal expansion coefficients through multiple layers, and good resistance to oxidation, erosion and corrosion. The CVD technique can be successfully used in depositing such multiple layers with excellent control of thickness and uniformity.
449
2 Physical Vapour Deposition (PVD) Physical Vapour Deposition (PVD) is another coating technique. The reactants (precursors) are solids, which are forced in a gaseous state. This can be done by simple heating, but mostly, this procedure involves ion - bombing in order to create a plasma. The gaseous phase deposits on the solid substrate at relatively low temperatures. The main advantages of the PVD technique are: * the purity of the coating; * the broad range of suitable precursors; * the low temperature of the substrate; * the fine structure of the coating, which makes polishing unnecessary. The main disadvantages are: * the low deposition rate; * the complexity of the process and the equipment; * the high demands concerning the purity and cleanliness of the substrate' surface; * the line - o f - sight effects. The line-of-sight effect is the biggest disadvantage. It means that the part of the substrate, which is located directly above the source, will achieve a much thicker coating that other parts of the substrate. As a consequence, only very simple geometric forms can be coated uniformly by PVD, provided that either the substrate is rotated, or that multiple sources are present in strategic places in the reactor. PVD is a time consuming process" not only the deposition rate is relatively low, also severe precautions are necessary concerning the cleanliness of the substrate. This involves long pretreatment procedures. Depending on the way the solid precursor is volatized, three PVD methods are discerned" * thermal PVD * sputtering * ion-plating
450
2.1 Thermal Physical Vapour Deposition P VD Figure 13.6 shows the basic configuration of a thermal PVD reactor.
[
]S
\
Iv lip
Figure 13.6 Schematic representation of a thermal PVD reactor (S= substrate," V= source P= pumps).
Since in a thermal PVD experiment, the solid source material is volatized by simple heating, the choice of precursors is rather limited (melting temperature < 1850 K). Conventional thermal PVD is mostly used for the deposition of metals. PVD of a ceramic layer involves the introduction of an additional gas. However, the energy of the gas is much too low to form a ceramic reaction product. For example, the formation of TiN from Ti and N2 needs much higher temperatures than the normal PVD process temperatures. This supplemental activation of the gas mixture is established by placing a positive electrode in the gas volume. This technique is called Activated Reactive Evaporation (ARE) and is illustrated in figure 13.7.
451
II +
Is
I
TE
\
~
/
V
II
P
Figure 13.7 Activated reactive evaporation (G= gas supply," S= substrate; V= source; TE= positive electrode," P= pumps).
2.2 Sputtering If a solid or liquid at any temperature is subjected to bombardment by suitable high energy atomic particles (usually ions), it is possible for individual atoms to acquire enough energy via collision processes to escape from the surface. This means of causing ejection of atoms from the surface is called sputtering. The atoms ejected from the surface can be used in depositing a coating on a substrate (figure 13.8).
I I C
I
I
IT
Is lip
Figure 13.8 Sputtering (G= gasinlet," T= target," S= substrate," P= pumps).
452 In the magnetron-sputtering technique (figure 13.9), the target is subjected to a magnetic field. This causes an additional ionization of the plasma near the target, allowing lower target temperatures.
II
~
L--
M
I
!r
i
is il
P
Figure 13.9 Magnetron sputtering (G= gasinlet," M= magnetron; T= target; S= substrate; P= pumps).
2.3 Ion plating Ion plating is a process in which a substrate is subjected to ion bombardment both before and during the time it is being coated. The coating process used in ion plating is vacuum evaporation from resistance - heated sources. The vacuum procedures in ion plating are the same as those used in sputtering in that the system is first pumped to a good vacuum and argon gas is then admitted to establish a state of dynamic epuilibrium in the chamber. The chamber pressure remains constant with argon being pumped out of the chamber at the same rate as it is flowing into the chamber. Chamber pressures in the order of 5 Pa are fairly standard in ion plating. A typical ion plating system is depicted in figure 13.10. Radio-frequency power is applied to the substrate, as in sputter cleaning, for the purposes of generating a plasma and of causing ion bombardment on the substrate surface. After sputter-cleaning has removed the contaminants from the substrate to establish atomically clean surfaces, the evaporation source is activated to begin the coating process.
453
II a
§+
T
-I
I r +
T
IS II P
Figure 13.10 Ion plating (G= gasinlet; S= substrate," T= target," P= pumps). 3 Atomic Layer Epitaxy Chemical Vapour Deposition involves a very complex mixture of gases and/or ions, causing numerous uncontrollable side reactions. From a chemical point of view, these reactions are extremely difficult to monitor. A basic understanding of all reactions occurring in a CVD experiment is lacking at the present time. This was recknognized by Suntola ~ in the late 1970's, when he developed a new coating technique: Atomic Layer Epitaxy (ALE). Atomic layer epitaxy is a method for producing thin films and layers of single crystals one atomic layer at a time, utilizing a self-control obtained through saturating surface reactions. ALE is thus based on separate surface reactions between the growing surface and each of the components of the compound, one at a time. These components are supplied in the vapour phase, either as elemental vapours or as volatile compounds of the elements. ALE was originally developed to meet the needs of improved ZnS thin films and dielectric thin films for electroluminescent thin film display devices. As an illustration of the ALE process, figure 13.11 summarizes the basic sequences of ALE in two alternative ways for producing ZnS.
454
ooooo, 9 OO,o.
'I ~ CI + Surplus H2S [
Zn (a)
2Zn+S
Figure 13.11 the reactants.
-. 2ZnS
, I (b)
ZnCI + H S -. ZnS + 2HCI
ALE process for ZnS. (a) Zn and S as the reactants; (b) ZnCI2 and H:S as
In each reaction there are sites for only one monolayer to make a bond with the original surface. A basic condition for a successful ALE process is that the binding energy of a monolayer chemisorbed on a surface is higher than the binding energy of subsequent layers on top of the formed monolayer. The temperature of the substrate is used as the primary controlling parameter. It is adjusted low enough to keep the monolayer on the surface until reaction with the following reactants takes place, but high enough to re - evaporate any of the subsequent layers on the top of the monolayer. The control of the monolayer can further be influenced with the aid of a laser beam or other extra energy. The greater the difference between the bond energy of a monolayer and the bond energies of the subsequent layers, the better the self- controlling characteristics of the process. Several general reaction mechanisms can be discerned in the ALE process.
The
simplest reaction sequence is an elemental reaction, which can be represented as reaction (A).
B(s) + A(g) --, BA(s)
(A)
455 The reaction of elemental Zn and S to form a ZnS layer is a typical example of such reaction. However, there might be different reasons for using compounds of the elements A and B as the reactants, rather than the elemental vapours of A and B. The most important reason is that for many metals, the vapour pressure is (too) low, which makes it very difficult to prevent condensation of the element at low substrate temperatures. In this cases, compound exchange reactions are used:
B(s) + AX(g) --, B-AX(s) B-AX(s) + B Y ( g ) - , B-AB(s) + XY(g)
(B)
The reaction ZnC12 + H2S --* ZnS + 2HC1 is a typical example. In many cases, the compound AX is a metal halide and the compound BY is a hydride. However, also organometallic compounds are attractive alternatives as reactants in an ALE process, owing to their high vapour pressure. There is a large amount of information available on many important organometallic compounds, because of the wide use in the CVD technology. Originated in the late 1970's, ALE has become a widely used synthesis route for thin epitaxial layers. Many ALE coated materials are produced commercially today. A very extended review on the achievements of the ALE technique, written by the pioneer in the field, is found in reference 11.
4 Molecular Layering In Eastern Europe, parallel to the development of ALE, a quite similar coating technique was developed, called Molecular Layering. 12'13'~4'~5 The principles of Molecular Layering are based on the irreversible chemical reactions of the substrate with a functional reagent. In this way, monolayers can be created to be used as such or to undergo further reaction with a second reagent. Its principles are visualized in figure 13.12.
4~
a) layer of given thickness
pB
functionalgroups BB
BB
BB
~
/
B B B B B
~
~
\
+Ac2 + BC-
BB
solid body b) layers with given disposition of monolayers of different chemical nature /
+AC 4
c
cc
BB
\
/
BB
\
/
BB
k
/
C
B
CC CC C "(~ "(~/ "(1~/
cc, ,c + I~4
v
- BC ~
+
-BC c) muldcoml~nent monolayers with given C CC CC CC C
monolayer
mixture
C CC CC CC C \
I
\_.J
\_.../ \
I
+ (xAC4 + yNC 4 ) - BC, - zAC4
C~(~/C
B CN'C B
+ yNC,
-B---F-"
C CCCC C ~A)/ ~ ~
- ~, \ \' ~~\ ~~\ \'~~\ \ . ~ .~~ %
+ zMC4
BB
~"
- BC
"
Figure 13.12 Principles of the molecular layering technique, according to Malygin; taken from ref. (22) with permission.
457 An important application of this technique is the creation of thin transition metal oxides on a substrate, by reacting consecutively with a metalchloride (TIC14) or oxychloride (CRO2C12, VOC13) and water, according to reaction (C). 16'17'~g ( - Si-OH),, + TiCl 4
(~- Si-O).TiCl,.
+
~
(4-n)H20
( - - Si-O)nTi(OH)4_n
+
( - Si-O).TiCl4..
~
+
(-= Si-O).Ti(OH),.
TiC! 4
n HC!
+
(4-n)HCl
---> ( - - Si-O).(Ti.O)mTiCl4_ m
(~- Si-O).(Ti-O)mTiCl4_ m +
(4-m)H20
-,,
....
(C)
Another typical application is the creation of sensors. Vanadium modified silica, according to reaction (D), yields a very useful compound for the visual control of humidity in gas media. 19 (-- SiOH). + VOCI 3 ~
(---Si-O).VOCI3_ n + nHC!
(D)
The different colour changes, compared to commercial cobalt containing silica, are presented in table 13.5.
Table 13.5 Comparison of the sensitivity (colour changes) of V-modified silica (ML-method) and commercial Co-containing silica Relative humidity % Dew point K
0.5 233
1.6-4.6 6-10 243-253 255-260
10-13 261-263
15-45 264-279
48-95 281-292
colour V-silica of the external layer commerc, Co-silica
lemon
bright yellow
light orange
dark orange
from red brown to dark
blue
blue
light blue
light rosy
rosy
458
5 Chemical Surface Coating In the early 90's, a new technique, 'Chemical Surface Coating (CSC)' was developed by Vansant, Gillis-D'Hamers, Van Der Voort and Vrancken, in order to create thin ceramic layers on a substrate by successive chemical modifications. The principles and developments of this technique are the subject of a separate section of this book (cfr. chapter 14).
References
D.G. Bhat, A review of chemical vapour deposition-techniques, materials and applications; surface modification technologies, eds. T.S. Sudarshan and D.G. Bhat, 1988.
.
D
C.F. Powell, in Vapour Deposition, C.F. Powell, J.H. Oxley and J.M. Blocher jr. eds., John Wiley and Sons, New York, 1966, pp. 249-276.
3.
D.P. Dapkus, Ann. Rev. Mater. Sci., 1982, 12, 243.
4.
R.A. Holtz, R.E. Benander and R.D. Davis, U.S. Patent 4 427 445, 1984.
5.
N.J. Archer and K.K. Yee, Wear, 1978, 48, 237.
6.
R.B. Pratt et al., Nucl. Appl., 1969, 6, 241.
0
S. Lai, M.P. Dudokovic and P.A. Ramachandran, Chem. Eng. Sci., 1986, 41, 633.
8.
G.C. Hsu et al., NASA Tech. Brief., 1985, 9, 104.
9.
D.P. Stinton, A.J. Caputo and R.A. Lowden, Ceramic Bulletin, 1986, 65, 347.
10.
C. Hollabaugh, in Proceedings of sixth international conference on chemical vapour deposition, The Electrochemical Society Inc., Princeton, New Jersey, 1977, pp. 419-429.
11.
T. Suntola, Materials Science Reports, 1989, 4, 261 - 321 and references therein.
12.
A.A. Chuiko, Teoret. i. Eksperim. Khimiya, 1987, 5, 597.
13.
V.B. Aleskovskiy, Stehiometriya i sinteztverdyh soedinery, L/Nauka, 1976.
459 15.
S.I. Koltsov and V.B. Aleskovskiy, Zhurnal Prilk. Khimii, 1967, 40, 207.
16.
M.N. Tzetkova, I.M. Yur'evskaya and A.A. Malygin, Zhurn. Prikl. Khimii., 1982, 2, 256.
17.
S.D. Dubrovenskiy, A.E. Emelyanov and A.V. Zimin, Zhurn. Prikl. Khimii, 1992,
65, 2259. 18.
E.A. Avrutina, Avtoreferat diss. kandidat khim. nauka., Leningrad, 1989.
19.
A.A. Malygin, private communication.