The Materials Science of Thin Films 2011 Part 5 docx

High-Temperature Systems There is need to reduce semiconductor processing temperatures, but the growth of high-quality epitaxial thin films can only be achieved by high-temperature CVD

178 Chemical Vapor Deposition

B,H, mixtures generate BPSG As noted in Section 4.6.3, LPCVD processes have largely surpassed atmospheric CVD methods for depositing such films

4.6.2 High-Temperature Systems

There is need to reduce semiconductor processing temperatures, but the growth

of high-quality epitaxial thin films can only be achieved by high-temperature CVD methods This is true of Si as well as compound semiconductors High-temperature atmospheric systems are also extensively employed in metal- lurgical coating operations The reactors can be broadly divided into hot-wall and cold-wall types Hot-wall reactors are usually tubular in form, and heating

is accomplished by surrounding the reactor with resistance elements An example of such a reactor for the growth of single-crystal compound semicon- ductor films by the hydride process was given in Fig 4-3 Higher temperatures are maintained in the source and reaction zones ( - 800-850 "C) relative to the deposition zone (700 "C) Prior to deposition, the substrate is sometimes

4.6 CVD Processes and Systems 179

etched by raising its temperatures to 900 "C Provision for multiple tempera- ture zones is essential for efficient transport of matrix as well as dopant atoms

By programming flow rates and temperatures, the composition, doping level and layer thickness can be controlled, making it possible to grow complex multilayer structures for device applications

Cold-wall reactors are utilized extensively for the deposition of epitaxial Si films Substrates are placed in good thermal contact with Sic-coated graphite susceptors, which can be inductively heated while the nonconductive chamber walls are air- or water-cooled Three popular cold-wall reactor configurations are depicted in Fig 4-13 (Ref 23) Of note in both the horizontal and barrel reactors are the tilted susceptors This feature compensates for reactant deple- tion, which results in progressively thinner deposits downstream as previously discussed In contrast to the other types, the wafer substrates lie horizontal in the pancake reactor Incoming reactant gases flow radially over the substrates where they partially mix with the product gases Cold-wall reactors typically

operate with H, flow rates of 100-200 (standard liters per minute) and 1 vol%

of SiC1, Silicon crystal growth rates of 0.2 to 3 pm/min are attained under these conditions Substantial radiant heat loss from the susceptor surface and consumption of large quantities of gas, 60% of which is exhausted without reacting at the substrate, limit the efficiency of these reactors

4.6.3 Low-Pressure CVD

One of the more recent significant developments in CVD processing has been the introduction of low-pressure reactor systems for use in the semiconductor industry Historically, LPCVD methods were first employed to deposit polysil- icon films with greater control over stoichiometry and contamination problems

In practice, large batches of wafers, say 100 or more, can be processed at a time This coupled with generally high deposition rates, improved film thick- ness uniformity, better step coverage, lower particle density, and fewer pinhole defects has given LPCVD important economic advantages relative to atmo- spheric CVD processing in the deposition of dielectric films

The gas pressure of - 0.5 to 1 torr employed in LPCVD reactors distin- guishes it from conventional CVD systems operating at 760 torr To compen- sate for the low pressures, the input reactant gas concentration is correspond- ingly increased relative to the atmospheric reactor case Low gas pressures primarily enhance the mass flux of gaseous reactants and products through the boundary layer between the laminar gas stream and substrates According to

Eq 4-3 1, the mass flux of the gaseous specie is directly proportional to D / 6

180 Chemical Vapor Deposition

Since the diffusivity varies inversely with pressure, D is roughly lo00 times higher in the case of LPCVD This more than offsets the increase in 6, which

is inversely proportional to the square root of the Reynolds number In an LPCVD reactor, the gas flow velocity is generally a factor of 10-100 times higher, the gas density a factor of loo0 lower, and the viscosity unchanged relative to the atmospheric CVD case Therefore, Re is a factor of 10 to 100 times lower, and 6 is about 3 to 10 times larger Because the change in I)

dominates that of 6, a mass-transport enhancement of over an order of magnitude can be expected for LPCVD The increased mean-free path of the gas molecules means that substrate wafers can be stacked closer together, resulting in higher throughputs When normalized to the same reactant partial pressure, LPCVD film growth rates exceed those for conventional atmospheric CVD

The commercial LPCVD systems commonly employ horizontal hot-wall reactors like that shown in Fig 4-14 These consist of cylindrical quartz tubes

heated by wire-wound elements Large mechanical pumps as well as blower

booster pumps are required to accommodate the gas flow rates employed-e.g., 50-500 standard cm3/min at 0.5 torr-and maintain the required operating pressure One significant difference between atmospheric and LPCVD systems concerns the nature of deposition on reactor walls Dense adherent deposits accumulate on the hot walls of LPCVD reactors, whereas thinner, less adherent films form on the cooler walls of the atmospheric reactors In the latter case, particulate detachment and incorporation in films is a problem, especially on horizontally placed wafers It is less of a problem for LPCVD reactors where vertical stacking is employed Typically, 100 wafers, 15 cm in

4.6 CVD Processes and Systems 181

diameter, can be processed per hour in this reactor In addition to polysilicon and dielectric films, silicides and refractory metals have been deposited by LPCVD methods

4.6.4 Plasma-Enhanced CVD

In PECVD processing, glow discharge plasmas are sustained within chambers where simultaneous CVD reactions occur The reduced-pressure environment utilized is somewhat reminiscent of LPCVD systems Generally, the radio frequencies employed range from about 100 kHz to 40 MHz at gas pressures

between 50 mtorr to 5 torr Under these conditions, electron and positive-ion densities number between lo9 and 101*/cm3, and average electron energies range from 1 to 10 eV This energetic discharge environment is sufficient to decompose gas molecules into a variety of component species, such as elec- trons, ions, atoms, and molecules in ground and excited states, free radicals, etc The net effect of the interactions among these reactive molecular frag- ments is to cause chemical reactions to occur at much lower temperatures than

in conventional CVD reactors without benefit of plasmas Therefore, previ- ously unfeasible high-temperature reactions can be made to occur on tempera- ture-sensitive substrates

In the overwhelming majority of the research and development activity in PECVD processing, the discharge is excited by an rf field This is due to the

R F

A L U M I N U M

ELECTRODE

PUMP N H 3 (+*;*OR) Figure 4-1 5

Ref 26)

Typical cylindrical, radial flow, silicon nitride deposition reactor (From

182 Chemical Vapor Deposition

fact that most of the films deposited by this method are dielectrics, and dc discharges are not feasible The tube or tunnel reactors employed can be coupled inductively with a coil or capacitively with electrode plates In both cases, a symmetric potential develops on the walls of the reactor High wall potentials are avoided to minimize sputtering of wall atoms and their incorpo- ration into growing films

A major commercial application of PECVD processing has been to deposit silicon nitride films in order to passivate and encapsulate completely fabricated microelectronic devices At this stage the latter cannot tolerate temperatures much above 300 "C A parallel-plate, plasma deposition reactor of the type shown in Fig 4-15 is commonly used for this purpose The reactant gases first flow through the axis of the chamber and then radially outward across rotating substrates that rest on one plate of an rf-coupled capacitor This diode configuration enables a reasonably uniform and controllable film deposition to occur The process is carried out at low pressures to take advantage of enhanced mass transport, and typical deposition rates of about 300 i / m i n are attained at power levels of 500 W Silicon nitride is normally prepared by reacting silane with ammonia in an argon plasma, but a nitrogen discharge with

Table 4-3 Physical and Chemical Properties of Silicon Nitride Films

6-7

10' 1015- 1017 1.5 x 10" (T)

None Zero Excellent

0.75

80 i / m i n Fair

< l o o i

2.9-3.1 2.5-2.8 2.01 2.0-2.1

1500-3000 i / m i n Conformal

0.75 0.8- 1 .O

< l 0 0 i

Note: T = tensile; C = compressive

4.6 CVD Processes and Systems 183 Table 4-4 PECVD Reactants and Products, Deposition Temperatures, and Rates Deposit T (K) Rate (cm/sec) Reactants

< 313

313

1013-1213 313-513

523

523

1213 523-113 413-613

513-113

1213

813 523-1213 613-913 633-613

413-113 613-813

GeH,-H,; Ge(s)-H, B,H,; BCI,-H,; BBr, P(s)-H,

ASH,; As(s)-H, Me-H , Mo(CO), NKCO), C(s)-H,; C(s)-N, Cd-HzS Si(OC,H,),; SiH,-O,, N,O Ge(OC,H,),; GeH,-O,, N,O SiCI,-GeC14 + 0 ,

AIC13-0, TiC1,-0, ; metallorganics B(OC,H,),-O,

SiH,-N,, NH GaCI,-N, TiCI,-H, + N, AICI,-N,

B,H6-NH3 P(s-N, ; PH 3-NZ SiH,-C,H, TiC14-CH4 + H, BZHG-CH,

From Ref 27

silane can also be used As much as 25 at % hydrogen can be incorporated in plasma silicon nitride, which may, therefore, be viewed as a ternary solid solution This should be contrasted with the stoichiometric compound Si,N, , formed by reacting silane and ammonia at 900 "C in an atmospheric CVD reactor It is instructive to further compare the physical and chemical property

differences in three types of silicon nitride, and this is done in Table 4-3

Although Si,N, is denser, more resistant to chemical attack, and has higher resistivity and dielectric breakdown strength, SiNH tends to provide better step coverage

184 Chemical Vapor Deposition

Some elements, such as carbon and boron, in addition to metals, oxides,

nitrides, and silicides, have been deposited by PECVD methods Operating

temperatures and nominal deposition rates are included in Table 4 4 An

important recent advance in PECVD relies on the use of microwave-also

called electron cyclotron resonance (ECR)-plasmas As the name implies, microwave energy is coupled to the natural resonant frequency of the plasma electrons in the presence of a static magnetic field The condition for energy absorption is that the microwave frequency w , (commonly 2.45 GHz) be equal

to q B / m , where all terms were previously defined in connection with magnetron sputtering (Section 3.7.3) Physically, plasma electrons then un- dergo one circular orbit during a single period of the incident microwave Whereas rf plasmas contain a charge density of - 10" cm-3 in a 10-2-to-l- torr environment, the ECR discharge is easily generated at pressures of

to torr Therefore, the degree of ionization is about loo0 times higher than in the rf plasma This coupled with low-pressure operation, controllability

of ion energy, low-plasma sheath potentials, high deposition rates, absence of source contamination (no electrodes!), etc., has made ECR plasmas attractive for both film deposition as well as etching processes A reactor that has been

employed for the deposition of S O , , Al,O, , SiN, and Ta,05 films is shown

in Fig 4-16 A significant benefit of microwave plasma processing is the ability to produce high-quality films at low substrate temperatures

MICROWAVE 2.45 GHz

GAS

SiHd

MAGNET COILS

Figure

Noyes Publications)

4-16 ECR plasma deposition reactor (From Ref 28, with permission from

4.6 CVD Processes and Systems 185

4.6.5 Laser-Enhanced CVD

Laser or, more generally, optical chemical processing involves the use of monochromatic photons to enhance and control reactions at substrates Two mechanisms are involved during laser-assisted deposition, and these are illus- trated in Fig 4-17 In the pyrolytic mechanism the laser heats the substrate to decompose gases above it and enhance rates of chemical reactions there Pyrolytic deposition requires substrates that melt above the temperatures necessary for gas decomposition Photolytic processes, on the other hand, involve direct dissociation of molecules by energetic photons Ultraviolet light sources are required because many useful parent molecules (e.g., SiH, ,

Si,H, , Si,H, , N,O) require absorption of photons with wavelengths of less than 220 nm to initiate dissociation reactions The only practical continuous- wave laser is the frequency-doubled Ar+ at 257 nm with a typical power of 20

mW Such power levels are too low to enable high deposition rates over large areas but are sufficient to “write” or initiate deposits where the scanned light beam hits the substrate Similar direct writing of materials has been accom- plished by pyrolytic processes Both methods have the potential for local deposition of metal to repair integrated circuit chips

A number of metals such as Al, Au, Cr, Cu, Ni, Ta, Pt, and W have been

L A S E R - ASS I STED DEPOSl T ION

LASER

I / B E A M

P Y R O L Y T I C A

SUBSTRATE REG ION

Figure 4-1 7

from Ref 29, 0 1985 by Annual Reviews Inc.)

Mechanisms of laser-assisted deposition (Reproduced with permission

186 Chemical Vapor Deposition

deposited through the use of laser processing For photolytic deposition, organic metal dialkyl and trialkyls have yielded electrically conducting de- posits Carbonyls and hydrides have been largely employed for pyrolytic depositions There is frequently an admixture of pyrolytic and photolytic deposition processes occurring simultaneously with deep UV sources Alterna- tively, pyrolytic deposition is accompanied by some photodissociation of loosely bound complexes if the light source is near the UV

Dielectric films have also been deposited in low-pressure photosensitized CVD processes (Ref 30) The photosensitized reaction of silane and hydrazine yields silicon nitride films, and SiO, films have been produced from a gas mixture of SiH,, N,O, and N, In SiO,, deposition rates of 150 A/rnin at temperatures as low as 50 "C have been reported (Ref 23), indicating the exciting possibilities inherent in such processing

4.6.6 Metalorganic CVD (MOCVO) (Ref 31)

Also known as OMVPE (organometallic vapor phase epitaxy), MOCVD has presently assumed considerable importance in the deposition of epitaxial compound semiconductor films, Unlike the previous CVD variants, which differ on a physical basis, MOCVD is distinguished by the chemical nature of the precursor gases As the name implies, metalorganic compounds like trimethyl-gallium (TMGa), trimethyl-indium (TMIn), etc, are employed They are reacted with group V hydrides, and during pyrolysis the semiconductor compound forms; e.g.,

(4-51)

Group V organic compounds TMAs, TEAS (triethyl-arsenic), TMP, TESb, etc., also exist, so that all-organic pyrolysis reactions have been carried out The great advantage of using metalorganics is that they are volatile at moder- ately low temperatures; there are no troublesome liquid Ga or In sources in the reactor to control for transport to the substrate Carbon contamination of films

is a disadvantage, however Since all constituents are in the vapor phase, precise electronic control of gas flow rates and partial pressures is possible

This, combined with pyrolysis reactions that are relatively insensitive to temperature, allows for efficient and reproducible deposition Utilizing com- puter-controlled gas exchange and delivery systems, epitaxial multilayer semi- conductor structures with sharp interfaces have been grown in reactors such as shown in Fig 4-18 In addition to GaAs, other 111-V as well as 11-VI and

IV-VI compound semiconductor films have been synthesized Table 4-5 lists

4.6 CVD Processes and Systems 187

VACUUM

FLASK Figure 4-1 8

tor (Reprinted with permission From R D Dupuis, Science 226, 623, 1984)

Schematic diagram of a vertical atmospheric-pressure MOCVD reac-

Table 4-5 Organo Metallic Precursors and Semiconductor Films

1 a8 Chemical Vapor Deposition

some films formed on insulating and semiconducting substrates together with corresponding reactants and film growth temperatures

Film growth rates (6) and compositions directly depend on gas partial pressures and flow rates ( V ) For Al,Ga, -,As films,

(4-51)

(4-52)

In these equations K ( T) is a temperature-dependent constant, and the factor of

2 enters because trimethyl-aluminum is a dimer MOCVD has been particu- larly effective in depositing films for a variety of visible and long-wavelength lasers as well as quantum well structures The use of these precursor gases is not only limited to semicondiictor technology; volatile organo-Y, Ba, and Cu compounds have been explored in connection with the deposition of high-tem- perature superconducting films having the nominal composition YBa,Cu 307

4.6.7 Safety

The safe handling of gases employed in CVD systems is a concern of paramount importance Because the reactant or product gases are typically toxic, flammable, pyrophoric, or corrosive, and frequently possess a combina- tion of these attributes, they present particular hazards to humans Exposure of reactor hardware and associated gas-handling equipment to corrosive environ- ments also causes significant maintenance problems and losses due to down- time Table 4-6 lists gases commonly employed in CVD processes together with some of their characteristics A simple entry in the table does not accurately reflect the nature of the gas in practice Silane, for example, more

so than other gases employed in the semiconductor industry, has an ominous and unpredictable nature It is stable but pyrophoric, so it ignites on contact with air If it accumulates in a stagnant airspace, however, the resulting mixture may explode upon ignition In simulation tests of leaks, high flow rates of silane have resulted in violent explosions For this reason, silane cylinders are stored outside buildings in concrete bunkers The safety problems are magnified in low-pressure processing where concentrated gases are used For example, in the deposition of polysilicon, pure silane is used during LPCVD, whereas only 3% silane is employed in atmospheric CVD processing Corrosive attack of gas-handling equipment (e.g., valves, regulators, piping) occurs in virtually all CVD systems The problems are particularly acute in LPCVD processing because of the damage to mechanical pumping systems

4.6 CVD Processes and Systems

Table 4-6 Hazardous Gases Employed in CVD

Bodily Gas Corrosive Flammable Pyrophoric Toxic Hazard

x Anemia, kidney damage death

x Eyeand respiratory irritation

x Respiratory irritation

X

Severe burns

x Respiratory irritation, death

X

X

190 Chemical Vapor Deposition

Since many reactors operate at high temperatures, the effluent gases are very hot and capable of further downstream reactions in the pumping hardware Furthermore, the exhaust stream generally contains corrosive species such as acids, water, oxidizers, unreacted halogenated gases, etc., in addition to large quantities of abrasive particulates In semiconductor processing, for example, SiO, and Si,N, particles are most common All of these products are ingested

by the mechanical pumps, and the chamber walls become coated with precipi- tates or particulate crusts The oils used are degraded through polymerization and incorporation of solids The lubrication of moving parts and hardware is thus hampered, and they tend to corrode and wear out more readily All of this

is a small price to pay for the wonderful array of film materials that CVD has made possible

Repeat parts (a) and (b) for VC films from a VCl, + C,H,CH3 + H,

gas mixture

H ,

2 Consider the generic reversible CVD reaction

A, 2 B, + Cg(T, > T,)

at 1 atm pressure (PA + Pc = l ) , where the free energy of the reaction is

AGO = A H " - T A S " Through consideration of the equilibria at T,

the sign and value of A H

3 In growing epitaxial Ge films by the disproportionation reaction of Eq

Exercises 191

4-13, the following thermodynamic data apply:

I,(,, = 21(,,

Ge(s) + I,(,, = GeIq,,

Ge(,, + GeI,(,, = 2GeI,,,,

AGO = -38.4T cal/mole

AG" = - 1990 - 11.2T cal/mole

AGO = 36300 - 57.5T cal/mole

a What is AGO for the reaction Ge(,, + 21,,,, = GeI,(,,?

b Suggest a reactor design Which region is hotter; which is cooler?

c Roughly estimate the operating temperature of the reactor

d Suggest how you would change the reactor conditions to deposit polycrystalline films

4 a At 1200 "C the following growth rates of Si films were observed using the indicated Si-C1-H precursor gas The same CVD reactor was employed for all gases

Precursor Growth Rate (pn/min)

3 x lo6 cm-' for SiHCl,

Are the observations made in (a) and (b) consistent? From what you know about these gases explain the two findings

5 Plot lnP&, /Psicl,P& vs 1/ T K for the temperature range 800 to 1500

K, using the results of Fig 4-5

a What is the physical significance of the slope of this Arrhenius plot?

b Calculate A H for the reaction given by Eq 4-22a, using data in Fig 4-4

6 Assume you are involved in a project to deposit ZnS and CdS films for infrared optical coatings Thermodynamic data reveal

192 Chemical Vapor Deposition

b In practice, reactions 1 and 2 are carried out at 680 "C and 600 'C, respectively From the vapor pressures of Zn and Cd at these tempera- tures, estimate the PH, / P H Z s ratio for each reaction, assuming equi- librium conditions

c Recommend a reactor design to grow either ZnS or CdS, including a

method for introducing reactants and heating substrates

7 It is observed that when WF, gas passes over a substrate containing exposed areas of Si and SiO, :

1 W selectively deposits over Si and not over SiO,

2 Once a continuous film of W deposits (i.e., - 100-150 A), the Suggest a possible way to subsequently produce a thicker W deposit

8 The disproportionation reaction Si + SiCl, = 2SiC1, (AGO = 83,000 +

3.64T log T - 89.4T (cal/mole)) is carried out in a closed tubular atmospheric pressure reactor whose diameter is 15 cm Deposition of Si occurs on a substrate maintained at 750 "C and located 25 cm away from the source, which is heated to 900 "C Assuming thermodynamic equilib- rium prevails at source and substrate, calculate the flux of SiCl, trans- ported to the substrate if the gas viscosity is 0.08 cP [Hint: See problem reaction is self-limiting and no more W deposits

2.1

9 Find the stoichiometric formula for the following films:

a PECVD silicon nitride containing 20 at% H with a Si/N ratio of 1.2

b LPCVD silicon nitride containing 6 at% H with a Si/N ratio of 0.8

c LPCVD SiO, with a density of 2.2 g/cm3, containing 3 x 10'' H

atoms/cm3

lo Tetrachlorosilane diluted to 0.5% mole in H, gas flows through a 12-cm-diameter, tubular, atmospheric CVD reactor at a velocity of 20 cm/sec Within the reactor is a flat pallet bearing Si wafers resting horizontally If the viscosity of the gas is 0.03 CP at 1200 "C,

a what is the Reynolds number for the flow?

b estimate the boundary layer thickness at a point 5 cm down the pallet

c If epitaxial Si films deposit at a rate of 1 pm/min, estimate the

11 Polysilicon deposits at a rate of 30 i / r n i n at 540 "C What deposition rate can be expected at 625 "C if the activation energy for film deposition

is 1.65 eV?

diffusivity of Si through the boundary layer

References 193

12 Consider a long tubular CVD reactor in which one-dimensional steady- state diffusion and convection processes occur together with a homoge- neous first-order chemical reaction Assume the concentration C( x) of a given species satisfies the ordinary differential equation

13 Select any film material (e.g., semiconductor oxide, nitride carbide metal alloy etc.) that has been deposited or grown by both PVD and CVD methods In a report compare the resultant structures stoichiome- tries and properties The Journal of Vacuum Science and Technology

and Thin Solid Films are good references for such information

REFERENCES

1 * W Kern and V S Ban in Thin Film Processes eds J L Vossen and

2 * W Kern in Microelectronic Materials and Processes, ed R A Levy 3.* K K Yee Int Metals Rev 23, 19 (1978)

4.* J W Hastie High Temperature Vapors - Science and Technology 5.* J M Blocher in Deposition Technologies f o r Films and Coatings,

6.* W A Bryant, J Mat Sci 12 1285 (1977)

7 * K K Schuegraf Handbook of Thin-Film Deposition Processes and

8

W Kern Academic Press, New York (1978)

Kluwer Academic Dordrecht (1989)

Academic Press, New York (1975)

ed R F Bunshah Noyes Park Ridge NJ (1982)

Techniques Noyes Park Ridge NJ (1988)

J Schlichting Powder Metal Int 12(3) 141 (1980)

*Recommended o r reviews

194 Chemical Vapor Deposition

T Mizutani, M Yoshida, A Usui, H Watanabe, T Yuasa, and

I Hayashi, Japan J Appl Phys 19, L113 (1980)

R A Laudise, The Growth of Single Crystals, Prentice Hall, Engle-

wood Cliffs, NJ (1970)

0 Kubaschewski and E L Evans, Metallurgical Thermochemistry,

Pergamon Press, New York (1958)

D R Stull and H Prophet, JANAF Thermochemical Tables, 2nd ed.,

U.S GPO, Washington, DC (1971)

V S Ban and S L Gilbert, J Electrochem SOC 122(10), 1382 (1975)

E Sirtl, I P Hunt and D H Sawyer, J Electrochem SOC 121, 919 (1974)

J E Doherty, J Metals 28(6), 6 (1976)

T C Anthony, A L Fahrenbruch and R H Bube, J Vac Sci Tech

A2(3), 1296 (1984)

P C Rundle, Int J Electron 24, 405 (1968)

A S Grove, Physics and Technology of Semiconductor Devices,

Wiley, New York (1967)

W S Ruska, Microelectronic Processing, McGraw-Hill, New York

(1987)

J Bloem and W A P Claassen, Philips Tech Rev 41, 60 (1983, 1984)

R B Marcus and T T Sheng, Transmission Electron Microscopy of

Silicon VLSl Circuits and Structures, Wiley, New York (1983)

S M Sze, Semiconductor Devices - Physics and Technology, Wiley,

New York (1985)

A C Adams, in VLSI Technology, 2nd ed., ed S M Sze, McGraw- Hill, New York (1988)

J R Hollahan and S R Rosler, in Thin Film Processes, ed J L

Vossen and W Kern, Academic Press, New York (1978)

M J Rand, J Vuc Sci Tech 16(2), 420 (1979)

S Veprek, Thin Solid Films 130, 135 (1985)

Chapter 5

-Film Formation and Structure

5.1 INTRODUCTION

Interest in thin-film formation processes dates at least to the 1920s During

research at the Cavendish Laboratories in England on evaporated thin films,

the concept of formation of nuclei that grew and coalesced to form the film was

advanced (Ref 1) All phase transformations, including thin-film formation,

involve the processes of nucleation and growth During the earliest stages of

film formation, a sufficient number of vapor atoms or molecules condense and

establish a permanent residence on the substrate Many such film birth events

occur in this so-called nucleation stage Although numerous high-resolution

transmission electron microscopy investigations have focused on the early

stages of film formation, it is doubtful that there is a clear demarcation

between the end of nucleation and the onset of nucleus growth The sequence

of nucleation and growth events can be described with reference to the

micrographs of Fig 5-1 Soon after exposure of the substrate to the incident

vapor, a uniform distribution of small but highly mobile clusters or islands is

observed In this stage the prior nuclei incorporate impinging atoms and

subcritical clusters and grow in size while the island density rapidly saturates

The next stage involves merging of the islands by a coalescence phenomenon

195

H 006

96 C

5.1 Introduction 197

ISLAND

STRANSKI - KRASTANOV

Figure 5-2 Basic modes of thin-film growth

that is liquidlike in character especially at high substrate temperatures Coales- cence decreases the island density, resulting in local denuding of the substrate where further nucleation can then occur Crystallographic facets and orienta- tions are frequently preserved on islands and at interfaces between initially disoriented, coalesced particles Coalescence continues until a connected net- work with unfilled channels in between develops With further deposition, the channels fill in and shrink, leaving isolated voids behind Finally, even the voids fill in completely, and the film is said to be continuous This collective set of events occurs during the early stages of deposition, typically accounting for the first few hundred angstroms of film thickness

The many observations of film formation have pointed to three basic growth modes: (1) island (or Volmer-Weber), (2) layer (or Frank-van der Merwe),

and (3) Stranski-Krastanov, which are illustrated schematically in Fig 5-2 Island growth occurs when the smallest stable clusters nucleate on the substrate and grow in three dimensions to form islands This happens when atoms or molecules in the deposit are more strongly bound to each other than to the substrate Many systems of metals on insulators, alkali halide crystals, graphite, and mica substrates display this mode of growth

The opposite characteristics are displayed during layer growth Here the extension of the smallest stable nucleus occurs overwhelmingly in two dimen- sions resulting in the formation of planar sheets In this growth mode the atoms are more strongly bound to the substrate than to each other The first complete monolayer is then covered with a somewhat less tightly bound second layer Providing the decrease in bonding energy is continuous toward the bulk crystal

value, the layer growth mode is sustained The most important example of this

growth mode involves single-crystal epitaxial growth of semiconductor films, a subject treated extensively in Chapter 7

198 Film Formation and Structure

The layer plus island o r Stranski-Krastanov (S.K.) growth mechanism is an intermediate combination of the aforementioned modes In this case, after forming one or more monolayers, subsequent layer growth becomes unfavor- able and islands form The transition from two- to three-dimensional growth is not completely understood, but any factor that disturbs the monotonic decrease

in binding energy characteristic of layer growth may be the cause For example, due to film-substrate lattice mismatch, strain energy accumulates in the growing film When released, the high energy at the deposit-intermediate- layer interface may trigger island formation This growth mode is fairly common and has been observed in metal-metal and metal-semiconductor systems

At an extreme far removed from early film formation phenomena is a regime o f structural effects related to the actual grain morphology of polycrys- talline films and coatings This external grain structure together with the internal defect, void, or porosity distributions frequently determines many of the engineering propcrties of films For example, columnar structures, which interestingly develop in amorphous as well as polycrystalline films, have a profound effect on magnetic, optical, electrical, and mechanical properties In this chapter we discuss how different grain and dcposit morphologies evolve as

a function of deposition variables and how some measure of structural control can be exercised Modification of the film structure through ion bombardment

o r laser processing both during and after deposition has been a subject of much research interest recently and is treated in Chapters 3 and 13 Subse- quent topics in this chapter are:

5.2 Capillarity Theory

5.3 Atomistic Nucleation Processes

5.4 Cluster Coalescence and Depletion

5.5 Experimental Studies of Nucleation and Growth

5.6 Grain Structure of Films and Coatings

5.7 Amorphous Thin Films

References 1-5 are recommended sources for much of the subject matter in this chapter

5.2 CAPILLARITY THEORY

5.2.1 Thermodynamics

Capillarity theory possesses the mixed virtue of yielding a conceptually simple

qualitative model of film nucleation, which is, however, quantitatively inaccu-

5.2 Capillarity Theory 199

rate The lack of detailed atomistic assumptions gives the theory an attractive broad generality with the power of creating useful connections between such variables as substrate temperature, deposition rate, and critical film nucleus size An introduction to the thermodynamic aspects of homogeneous nucle- ation was given on p 40 and is worth reviewing In a similar spirit, we now consider the heterogeneous nucleation of a solid film on a planar substrate Film-forming atoms or molecules in the vapor phase are assumed to impinge

on the substrate, creating aggregates that either tend to grow in size or disintegrate into smaller entities through dissociation processes

The free-energy change accompanying the formation of an aggregate of mean dimension r is given by

AG = u3r3 AGv + u,r2yuf + a2r2yfs - a 2 r 2 T ~ ~

(5-1)

The chemical free-energy change per unit volume, A G v , drives the condensa-

tion reaction According to Eq 1-39, any level of gas-phase supersaturation

generates a negative AG, without which nucleation is impossible There are several interfacial tensions, y, to contend with now, and these are identified by the subscripts f, s, and u representing film, substrate, and vapor, respec- tively For the cap-shaped nucleus in Fig 5-3, the curved surface area (u,r2), the projected circular area on the substrate ( u2 r2), and the volume ( u3 r 3, are involved, and the respective geometric constants are a, = 27r(l - cos e ) ,

u2 = T sin28, a3 = 7r(2 - 3 cos 8 + cos38)/3 Consideration of the mechani- cal equilibrium among the interfacial tensions or forces yields Young’s equa- tion

Therefore, the contact angle 8 depends only on the surface properties of the involved materials The three modes of film growth can be distinguished on the

200 Film Formation and Structure

basis of Eq 5-2 For island growth, 0 > 0, and therefore

Ysu < Yfs + Y u f

Ysu = Yfs + Yuf

(5-3)

(5-4) For layer growth the deposit “wets” the substrate and 6 = 0 Therefore,

A special case of this condition is ideal homo- or autoepitaxy Because the

interface between film and substrate essentially vanishes, yfs = 0 Lastly, for S.K growth,

In this case, the strain energy per unit area of film overgrowth is large with respect to y U f , permitting nuclei to form above the layers In contrast, a film strain energy that is small compared with y u f is characteristic of layer growth Returning now to Eq 5-1, we note that any time a new interface appears there is an increase in surface free energy, hence the positive sign for the first two surface terms Similarly, the loss of the circular substrate-vapor interface under the cap implies a reduction in system energy and a negative contribution

to AG The critical nucleus size r* (Le., the value of r when d A G / d r = 0)

is given by differentiation, namely,

is easily shown that AG* is essentially the product of two factors; Le.,

The first is the value for AG* derived for homogeneous nucleation It is modified by the second term, a wetting factor that has the value of zero for

0 = 0 and unity for = 180” When the film wets the substrate, there is no

would be appropriate In the calculation for AG*, the denominator of Eq 5-7

would then be altered to 27a:(AGv + AG,)' Because the sign of AG,, is negative while AG,7 is positive, the overall energy barrier to nucleation

increases in such a case If, however, deposition occurred o n an initially strained substrate-i.e., one with emergent cleavage steps or screw disloca- tions-then stress relieval during nucleation would be manifested by a reduc- tion of AG* Substrate charge and impurities similarly influence AG* by

affecting terms related to either surface and volume electrostatic, chemical, etc., energies

5.2.2 Nucleation Rate

The nucleation rate is a convenient synthesis of terms that describes how many nuclei of critical size form on a substrate per unit time Nuclei can grow through direct impingement of gas phase atoms, but this is unlikely in the earliest stages of film formation when nuclei are spaced far apart Rather, the rate at which critical nuclei grow depends on the rate at which adsorbed

monomers (adatoms) attach to it In the model of Fig 5-3, energetic vapor

atoms that impinge on the substrate may immediately desorb, but usually they

remain on the surface for a length of time r, given by

phase Changes in Ede5 are particularly expected at substrate heterogeneities,

such as cleavage steps or ledges where the binding energy of adatoms is greater relative to a planar surface The proportionately large number of

atomic bonds available at these accommodating sites leads to higher Edrs

values For this reason, a significantly higher density of nuclei is usually

202 Film Formation and Structure

obsrI-\.ed near cleavage steps and other substrate imperfections The presence

of impurities similarly alters Edr, in a complex manner depending on type and distribution o f atoms or molecules involved

We now exploit some of these microscopic notions in the capillarity theory

of the nucleation rate N Reproducing Eq 1-41, we obtain the expression for N :

(The Zeldovich factor, included in other treatments, is omitted here for simplicity .) Based on the thermodynamic probability of existence, the equilib- rium number of nuclei of critical size per unit area of substrate is given by

N* = n,exp - A G * / k T (5-1 1) The quantity n, represents the total nucleation site density A certain number

of these sites are occupied by adatoms whose surface density, n o , is given by

the product of the vapor impingement rate (Eq 2-8) and the adatom lifetime,

on the substrate with a frequency given by v exp -E,/ k T , where E, is the

activation energy for surface diffusion The overall impingement flux is the product of the jump frequency and n o , or

rsPNAvexp - E , / k T ( c m P 2 s e c - ' )

(5-14)

There is no dearth of adatoms that can diffuse to and be captured by the existing nuclei During their residence time, adatoms are capable of diffusing a mean distance X from the site of incidence given by

E d e s - E,

X = a,exp