Techniques of Preparing Thin Films

2 Techniques of Preparing Thin Films

2.1 Introduction A film (also called a layer or coating) can be defined as a near-surface region whose properties are different from those of the bulk of the material. A thin film is a film having a maximum thickness of 1 μιη or less. However, manyfilms encountered in practical applications having thicknesses ranging from a few hundred Angstroms to several microns (typically 0.01 — 10 μιη) are generally considered as thin films. Because of the high technological importance of thin films in solid state electronics a wide variety of preparation techniques is presently available [1 — 10, 98—102]. These techniques may be classified according to the film formation environment: electrolysis (electroplating, electroless plating, and electrolytic anodisation); vacuum (vacuum evaporation, ion beam deposition, molecular beam epitaxy, hot-wall epitaxy, and ion implantation); plasma (sputter deposition and ion plating); liquid phase (liquid-phase epitaxy); solid phase (solid-phase epitaxy); and chemical vapour (substrate chemical vapour conversion and chemical vapour deposition). The techniques for preparing thin films have already been surveyed in a number of books and reviews. The intent of this chapter is to present the basic principle of all important techniques used to fabricate thin films. Typical apparatus and processing parameters, the advantages and limitations, as well as practical examples of representative applications in present-day solid-state technology of each technique are given. In this way, the similarities and differences between CVD and non-CVD methods will be better appreciated. However, the emphasis is placed on chemical vapour deposition (CVD), which is considered to be the major technique for preparing of most films used in the fabrication of semiconductor devices and integrated circuits. A direct comparison between CVD and non-CVD techniques is offered. 31

FUNDAMENTALS

2.2 Electrolytic Deposition Techniques Electroplating. Electroplating is a method of obtaining thin layers in an electrolytic solution under the action of the electric current, based on reducing the metallic ions at the cathode by introducing external electrons [103]. An electroplating deposition system consists of a plating bath filled with an electrolyte solution, a battery, and two electrodes (Fig. 2.1a). The anode is often made of the metal to be deposited and the cathode of the metal to be plated, while the electrolyte solution contains ions of the metal to be deposited. The electroplating process is affected by the following factors which control the deposition rate and the properties of electrodeposited film: the concentration and nature of the positive metal ions, the negatively charged ions, the organic and inorganic additives and impurities, the current density, the shape and surface of the cathode (substrate), and the temperature, agitation, pH, viscosity and surface tension of the solution. This method offers the advan-% tages of obtaining thin films at a relatively high deposition rate on substrates of various configurations using relatively simple equipment. However, it can only be applied to a limited number of materials, such as metals, which can be deposited only on a conductive substrate, the thickness uniformity being very low. Electroplating is used in solid state technology for the fabrication of thick copper films in order to increase the conductance of interconnections. Electroless plating. Electroless plating consists of the continuous formation of a metallic layer as a result of chemical reduction occurring in a plating solution, without employing an external current source [104—107].

(a)

*

(b)

(c)

5

Fig. 2.1 Equipment using chemical methods of film formation from solution: a — electroplating apparatus: 1 — electrolytic cell; 2 — electrolyte; 3 — anode; 4 — cathode being coated ; 5 — current source; b — electroless plating apparatus: 1 — electroless plating bath; 2 — plating solution; 3 — substrates having a catalytic surface immersed in the bath; 4 — substrate holder; 5 — heater; ■c — (wet) anodisation cell (after Dell'Oca and Barry [2236]; reprinted with permission from SOLID-STATE ELECTRONICS, Copyright© 1972, Pergamon Journals Ltd.): 1 — thermostatted bath; 2 — electrolyte (e.g., K N 0 3 in ethylene glycol for Si 3 N 4 anodisation, or tartaric acid solution for Al anodisation); 3 — stainless-steel cathode; 4 — mercury anode; 5 — wafer; 6 — vacuum; 7 — power supply; 8 — voltmeter; 9 — stirrer.

32

TECHNIQUES O F P R E P A R I N G T H I N F I L M S

Electroless plating is usually performed in a bath (Fig. 2Ab) containing the following components: a metallic salt providing the ions required for deposition, a reducing agent ensuring the reducing ability of the bath, a complexing agent preventing metallic ion precipitation, a pH controlling substance and a stabilizer reducing the homogeneous precipitation of the bath components. For Ni electroless plating the bath components are NiS0 4 , NaH 2 P0 2 , sodium citrate, NaOH, and thiourea. The important bath parameters are temperature, metallic and reducing ion concentrations, and pH. Unlike electroplating, this technique enables the preparation of denser, more uniform, and selective coatings on various non-conducting (dielectric, semiconducting) or metallic substrates. However, only a limited number of metals, alloys and semiconductor compounds can be deposited on non-conducting substrates. Since direct deposition cannot generally be obtained, activation of the substrate surface prior to deposition by means of immersion in solutions containing noble metals is usually required. Electroless plating has been applied mainly to Ni films on Si substrate. Electrolytic anodisation. Electrolytic anodisation (anodic oxidation) consists of producing an oxide coating by means of the electrochemical oxidation of an anode which forms the substrate material. The electrolytic anodisation equipment consists of an electrolytic cell, in which a voltage is applied between the anode and the cathode, which are immersed in a suitable electrolyte solution (Fig. 2.1c). The main process parameters are the current density, temperature and pH of the solution. This technique provides precise control of the oxide thickness by employing cheap equipment; but the oxides can only be formed on a limited number of substrates at room temperature and are contaminated and not very dense, their insulating properties being weak. Electrolytic anodisation is applied to growing anodic oxide layers on the surface of III—V compound semiconductors [108, 109].

2.3 Vacuum Deposition Techniques Vacuum evaporation. Vacuum evaporation consists of vaporizing the source material by heating it resistively, inductively or with an electron gun in vacuum, followed by its vapour recondensation as a thin film [110]. The equipment includes an evaporation enclosure containing heated supports on which the substrates are placed, an electron gun for heating the material powder, vacuum and temperature measuring instruments, and a vacuum pumping system (Fig. 22a). The main control parameters involved in evaporation are the source and substrate temperature, the source-substrate separation, and (in the case of reactive evaporation) the gas background pressure. This method is advantageous because deposition occurs even at room temperature on any kind of substrate. However, there are also some disadvantages that makes this method unsuitable for the deposition of high quality semiconductors and dielectrics: growth occurs very far from thermodynamic equilibrium; the substrate temperature is limited to 300—400°C owing to re-evaporation; undesired impurities originating from the evaporant support 33

FUNDAMENTALS

or from the walls of the vacuum chamber are introduced in the layers; crystalline defects are caused by radiation; non-uniform film thickness is caused by substrate sputtering and charge trapping in the deposited films produced by evaporant ionisation. Vacuum evaporation is specific to materials that are volatile at moderate temperatures, do not react with their support or bell jar materials at the evaporation temperature and that are not decomposed under the influence of evaporation conditions. In semiconductor technology, this technique is applied to device metallization (Al, Au) and to the fabrication of Ni—Cr resistors. Ion beam deposition. Thin layer deposition by means of ion beams can be obtained in two ways: the material can be deposited directly from an ion beam containing the desired element or compound, or indirectly by bombarding the target with external ions [120—127]. The direct deposition technique consists of the formation of a thin layer from the components of a low-energy ion beam reaching the substrate. Compared with conventional deposition methods such as vacuum evaporation, cathodic sputtering, and ion plating, direct ion beam deposition results in films without foreign impurities originating from the material source, equipment walls and gaseous ambient. However, the deposition time is much longer, and the equipment is more complex. The indirect technique is based on sputtering by using an external inert ion beam for removing the target material followed by its redeposition as a thin film on a substrate. A variant is ion beam reactive sputtering, in which an ion beam reacts with the target, the thin layer being either formed even on its surface or deposited on a substrate. The ion beam apparatus contains an ion beam source, a target holder, a substrate mount, and a vacuum chamber (Fig. 2.2b). The film deposition rate is mainly related to the type and energy of the impinging ions, the target material and temperature, the distance and angle of the substrate from the ion source and of the substrate from the target, and the substrate temperature. The main advantage of ion beam sputtering as compared to conventional sputtering is that, owing to working under high vacuum conditions, unwanted interactions between plasma and substrates can be avoided. However, disadvantages such as the need for a high vacuum with a low growth rate have so far prevented widespread use of this method. Ion beam sputtering has been applied to the deposition of some epitaxial semiconductor (Si, GaAs) and dielectric (Si3N4) films. Molecular beam epitaxy (MBE). Molecular beam epitaxy is a process involving interaction between a crystalline substrate surface and one or more molecular beams, obtained by electron beam or thermal evaporation of a selected source in an ultrahigh vacuum [128—133]. The MBE apparatus consists of a stainless steel bell jar, several source effusion cells, a heated substrate holder and an ion vacuum pump giving a typical pressure of ~ 1 X 10"l0 torr (Fig. 2.2c). The system may also be equipped with provisions for the in-situ study of the structure and the composition of the layers (LEED or RED and AES) and a quadrupole mass spectrometer for residual gas analysis and analysis of the molecular beams. The condensation process is mainly influenced by the nature of the effusing species, the incident molecular beam energy and flux, the substrate temperature (up to 650°C), the impurity doping beam flux, and the distance between the sources and the 34

TECHNIQUES OF P R E P A R I N G T H I N FILMS

substrate. Main advantages of this technique consist in their ability precisely to control the thickness, crystallinity, composition and impurity level of the elemental and compound epitaxial films, which are deposited monolayer by monolayer at relatively low temperatures. The disadvantages are low growth rate, limited crystal-thick layer capability,the requirement for an ultra-

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r—vVlw—1,.

6

(a)

(c)

Fig. 2.2 Equipment used in vacuum environment deposition techniques: a — vacuum evaporation equipment: 1 — bell jar; 2 — substrate holder with heater; 3 — substrate; 4 — source material; 5 — source heating filament; 6 — vacuum pump; b — ion-bearr sputtering deposition apparatus (after Weissmantel [123]; reprinted with permission from THIN SOLID FILMS, © 1976 Elsevier Sequoia S.A.): 1 — glass chamber; 2 — primary ion-beam source; 3 — secondary ion-beam source; 4 — target ; 5 — substrate holder; 6 — substrates; 7 — to vacuum; 8 — inert or reactive ion beam; 9 — sputtered material ; 10 — reactive ion beam; c — MBE system (after Cho and Arthur [128]; reprinted with permission from PROGRESS IN SOLID-STATE CHEMISTRY, (g) 1975 Pergamon Journals Ltd.): 1 - effusion cells with individual heat shielding; 2 — thermocouples; 3 — liquid-nitrogen shroud; 4 — mechanical shutter; 5 — molybdenum heating block; 6 — substrate; 7 — fluorescent screen; 8 — view port; 9 — quadrupole mass spectrometer; 10 — Auger cylindrical analyser; 11 — ion sputtering gun; 12 — electron diffraction gun; 13 — ultra-high vacuum chamber; d — HWE system (after Lopez-Otero [134]; reprinted with permission from THIN SOLID FILMS, Copyright© 1978 Elsevier Sequoia S.A.): 1 — quartz tube; 2 — substrate; 3 —source; 4 — substrate oven; 5 — wall oven; 6 — source oven;

35

FUNDAMENTALS

Έβ^ύ^Ο^^^όήI (e)

10

ir

Fig. 22e e — ion implantation system: 1 — source supplies; 2 — ion source; 3 — extraction a n d focusing electrodes; 4 — accelerating column; 5 — diffusion p u m p ; 6 — quadrupole lens; 7 — mass analyser; 8 — bsam scanner; 9 — ion detectors; 10 — sample chamber; 11 — wafer rack.

high vacuum, the fact that the process usually takes place under conditions very far from thermodynamic equilibrium (which could be detrimental to the epitaxial layers), and system expense. MBE has been applied especially to the construction of various heterostructures, such as multilayers of ALpGa^ As and GaAs that are thin enough (layer thickness as low as 10 A) to exhibit quantum size effects. Hot-wall epitaxy (RWE). Hot-wall epitaxy designates an epitaxial layer deposition process employing vacuum sublimation of a heated source followed by its condensation on a heated substrate, in a chamber using a heated liner (hot wall) which encloses and directs the vapour from the source to the substrate [134]. The equipment, shown schematically in Fig. 2.2d, is contained in a vacuum and consists of a quartz tube in which the source material is placed at the sealed bottom, while the substrate is placed on the open end at the top. The substrate, the wall of the tube and the source are resistively heated independently. The factors influencing the growth process are the source, the wall and the substrate temperatures , the geometry of the tube, the contamination of the source material, and the compensating sources, eventually used together with the main source material. This technique allows the preparation of thin layers having characteristics similar to those of the bulk materials, by working in thermal equilibrium and using relatively simple equipment. However, this technique is suitable only for sublimable solids and is limited to small-scale applications. HWE has been very successful in the preparation of epitaxial layers of II—VI and IV—VI compound semiconductors. Ion implantation. Ion implantation represents a method of obtaining thin layers which include atoms of the substrate [135—139]. It is a process in which suitable ions are injected at a certain depth beneath the substrate surface by bombarding it with high-energy accelerated ions. A thin layer is formed on subsequent thermal annealing of the substrate which facilitates the chemical combination of implanted and substrate atoms. The main parts of all ion implantation systems include an ion source, an accelerating and focusing column, a vacuum pump, a mass analyser, a beam scanner, an ion detector, and the sample chamber; these are presented schematically in Fig. 2.2£. The film thickness is controlled by the implantation voltage, the ion mass, the ion dose and the annealing temperature. By using this technique it is possible to obtain films with properties suitable for various 36

TECHNIQUES OF P R E P A R I N G T H I N FII.MS

applications, at room temperature. However, the crystalline lattice of the substrate is considerably damaged and the film rather thin, owing to the relatively shallow penetration of the ions. Ion implantation provides another method of forming thin films which incorporate surface atoms such as Si0 2 , SiaN4, SiC, Ge3N4, Ge0 2 , Ge^OyN-j, etc.

2.4 Plasma Deposition Techniques Sputter deposition. Sputter deposition, also named cathodic sputtering, is based on the process of neutral atom release from a cathodic target, bombarded with positive ionized gas molecules which are accelerated by means of an electric field, and their subsequent deposition on the substrate in order to form a thin solid film. A schematic diagram (Fig. 2.3#) of the cathodic sputtering deposition system shows a working enclosure provided with an anode on which the grounded or biased and heated or cooled substrates are positioned, a cathode with the sputtering material target, gas pressure measuring instruments, devices for introducing different gases and d.c, or preferable RF, high voltage sources. Depositions can be achieved in two ways: by direct cathodic sputtering (when the material to be sputtered is the same as the film obtained) and by reactive cathodic sputtering (when the

Ar

13

Fig. 2.3 Equipment used in plasma environment deposition techniques: a — multifunctional diode-type cathodic sputtering system: 1— d.c. source; 2— R F generator ; 3 — t a r g e t ; 4 — shutter; 5 — substrate; 6 — substrate holder; 7 — ground contact; 8 — d.c. bias; 9 — R F bias; 10 — bell jar chamber; 11 — to vaccum p u m p ; 12 — needle valve; 13 — inert or reactive gas inlet. b — R F ion plating system using electron-beam evaporation; 1 — vacuum chamber; 2 — substrate holder (water cooled cathode); 3 — substrate; 4 — source; 5 — electron-beam gun; 6 — R F power; 7 — R F matching network; 8 — shutter; 9 — R F glow discharge ; 10 — to vacuum pump.

37

FUNDAMENTALS

film results from the reaction between the target and certain gases introduced in the inert atmosphere). The main factores affecting the deposition process are the RF power, the target material, the substrate bias, the temperature, the reactive gas concentration in the inert sputtering gas, the shape and size of the electrodes, the chamber gas pressure and the presence of an auxiliary magnetic field. The method is advantageous because deposition can be accomplished using unheated sources, the substrates can be cleaned by ion bombardment before or during deposition and conformal coatings on irregular surface substrates can be obtained. However, the deposition rate is relative low, the film is contaminated from the target, deposition is only possible for materials which are available as plates, the film structure is damaged as a result of gas incorporation or bombardment with reactive species and energetic radiation, and the equipment is expensive. Sputtering has been successfully applied to deposit many films of resistors, conductors, insulators, semiconductors, magnets and superconductors [111 — 119]. Ion plating. Ion plating is a combination of vacuum evaporation with RF sputtering, being considered as evaporation in a glow discharge or evaporation with a biased substrate [140—142] . Similarly to sputtering, a gas plasma discharge is set up between a cathode (the substrate) and an anode (the source of the material to be deposited). Reactive ion plating uses in addition a reactive gas for preparing the compound films. In this process, the substrate is subjected to a flux of energetic ions which is sufficient for its sputter-etch cleaning before and during layer formation. Ion plating equipment (Fig. 2.3&) consists of a combination of a resistively or electron-beam heated evaporation system with a d.c. or RF plasma excited system. The process depends on the nature of the evaporation source, the substrate and reactive gas, the substrate temperature, the reactant gas pressure, the electron beam gun power, the RF power, the substrate voltage and the temperature. Although this technique includes the disadvantages inherent in each of both methods, it is characterized by a higher deposition rate, similar to that in vacuum evaporation, suitable substrate cleaning, good layer adhesion and the ability to achieve coatings on tridimensional substrates, as in the sputtering technique. A process of interest in solid-state technology is RF reactive ion plating of Si3N4.

2.5 Liquid-Phase Deposition Techniques Liquid-phase epitaxy. Liquid-phase epitaxy consists of precipitating a material from a cooling solution onto a heated substrate situated beneath [143—145]. The growth apparatus used in LPE is of two basic types: (a) the tipping furnace in which contact between the solution and the substrate is obtained by inclining the furnace (Fig. 2.4); and (b) the vertical system, in which the substrate is dipped into the solution. The LPE process depends on several factors, such as the purity of the gaseous ambients and the solution materials, the type of dopant additives, the growth temperature and the cooling rate. This technique offers many advantages, such as 38

T E C H N I Q U E S OF P R E P A R I N G T H I N F I L M S

Fig. 2.4 L P E growth apparatus (tipping furnace system) (after Nelson [144]; reprinted with permission from RCA Review): 1 — quartz t u b e ; 2 — furnace; 3 — graphite b o a t ; 4 — substrate; 5 — clamp; 6 — solution; 7 — thermocouple.

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u

**™

7

4 6"■■■

"W-H,

simplicity of equipment, high film deposition rates, easy incorporation in the layer of many useful dopants, and the elimination of chemical hazards. However, the reproducible preparation of some complex compounds is difficult, material stoichiometry cannot be adjusted easily owing to the presence of the solvent, the layer homogeneity h sometimes difficult to control, and the solvent adversely affects certain epitaxial layers. LPE has developed rapidly into a useful technique for preparing many binary and ternary III—V compounds and magnetic materials.

2.6 Solid-Phase Deposition Techniques Solid-phase epitaxy (SPE). Solid-phase epitaxy is based on conversion by thermal annealing at low temperature (400—600°C) of an amorphous layer deposited by using another technique (CVD or vacuum evaporation) in a layer having a monocrystalline structure similar to that of the substrate [147, 148]. The principle of this technique is schematically illustrated in Fig. 2.5. This technique provides a good thickness control of layers of high purity and crystallographic quality which are obtained at low temperatures using simple equipment. It is applicable to silicon and is compatible with present-day solid-state technologies. However, an adequately high cleanliness of the film-substrate interface which can be obtained by using UHV treatment, an intermediate Pd layer or even by an HF rinse before Si film deposition is usually required. An example is the SPE growth of Si from LPCVD polycrystalline films amorphized by means of 28Si ion implantation, which takes place by annealing in N2 at 550°C.

2.7. Chemical Vapour Conversion of Substrate Substrate conversion by using chemical vapour could be considered a special type of CVD process, in which the substrate is also participating in the heterogeneous deposition reaction as one of the reactants [149—179] However, Fig. 2.5 SPE growth equipment: 1 — quartz chamber; 2 — electric furnace; 3 — wafer carrier; 4 — wafer (e.g. single-crystalline silicon); 5 — evaporated metal layer (e.g. P d ) ; 6 — evaporated or CVD amorphous film (e.g. α-Si); 7 — wafer load /unload c a p ; 8 — vacuum system.

9 M I I I I T T /

A· I

/ '

—3 H I I U I I I I

39

FUNDAMENTALS

this method of film formation is in fact not a deposition process but rather an in-situ growth process. There are three main classes of substrate chemical vapour conversions according to the type of energy used in the process: thermal, plasma, and laser conversion. Substrate chemical vapour conversion, as applied to silicon technology, offers an extremely important advantage, namely the highest quality of grown Si0 2 , which is obtainable only through this technique. However, grown Si0 2 films have several disadvantages in planar semiconductor device processing over deposited Si0 2 films, e.g. some negative effects can be maximized owing to consumption of some of the silicon of the substrate (junction movement, pile-up or depletion of dopants at the Si02-Si interface); thick Si0 2 films useful in MOSFET circuits cannot be grown; these films are unable to serve as photolithographic masks for films not etched in fluorides, such as silicon nitride; and grown oxides do not lend themselves to in-siiu processing where two or more films are required and one of these films is SiO«. 2.7.1 Thermal Conversion of Substrate This technique comprises mainly thermal oxidation and thermal nitridation of elemental (Si, Ge) and compound semiconductors. Thermal oxidation. Thermal oxidation of silicon is the most appropriate process in many modern fabrication technologies used for building integrated circuits and discrete devices [149, 2078, 2079]. This process offers the advantages of reproducibility, obtaining very high quality uniform dielectric layers of silicon dioxide on a silicon wafer. Its disadvantages are that the high temperature (1000—1200°C) may lead to film contamination, the maximum oxide thickness obtained is about 1 μ, the duration of the process is considerable ( ^ 1 h), the substrate thickness is modified, and the process is inaplicable to semiconductor compounds. The proces usually proceeds at normal pressure, but at low or especially high pressure as well. Typical equipment for thermal oxidation of Si is shown in Fig. 2.6a. High-pressure oxidation. High pressure oxidation [2079] designates silicon oxidation at pressures up to 25 atmospheres in a variety of ambients (e.g. dry oxygen or steam environments) at temperatures between 500 and 1000°C. The main features of this process are acceleration of the oxidation process, denser and higher refractive index oxide formation, lower temperature (< 1000°C) processing (thus ensuring less wafer warping), less p-n junction delocalisation, fewer oxidation-induced stacking faults, and reduced dopant segregation at the Si—Si02 interface. However, high-pressure oxidation equipment is more complicated than atmospheric-pressure oxidation equipment (Fig. 2.66). Thermal nitridation. Thermal nitridation, i.e. thermal conversion of substrate surface to nitride layers — was studied for both silicon and compound semiconductors [150—154]. Thermal nitridation of silicon performed in highly purified nitrogen and ammonia gases at temperatures ranging from 900 to 1300°C gives a film with a thickness less than 100 Ä. The thermal nitridation process of silicon allows one to obtain a thinner insulator having 40

TECHNIQUES OF P R E P A R I N G T H I N F I L M S

good electrical interfacial and impurity masking properties, as required in advanced LSI processing. However, this process needs a relatively high temperature, and the film thickness is limited. Apparatus for the thermal nitridation of a silicon substrate is similar to that shown in Fig. 2.6a. "

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a,

N2

H2 HO

r^j-9

5

3

2

-w V±

(a)

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Fig. 2.6 Techniques and systems for chemical vapour substrate conversion: a — normal pressure thermal oxidation of silicon in dry oxygen (0 2 ), wet oxygen ( 0 2 — H 2 0 ) , steam (H 2 0), pyrogenic steam (H 2 —0 2 ) and HC1-containing ambients ( H O —O a , HC1—H 2 —0 2 ): 1 — membrane filters; 2 — flowmeters; 3 — two-way valves; 4 — water bubbler flask; 5 — three-way valve; 6 — v e n t ; 7 — quartz t u b e ; 8 — furnace; 9 — silicon wafers; 10 — quartz cradle; 11 — exhaust; b — high pressure oxidation in pyrogenic steam or oxygen (after Tsubouchi et al. [2143]; reprinted with permission from J A P A N E S E J O U R N A L OF A P P L I E D P H Y S I C S ) : 1 - q u a r t z t u b e ; 2 — furnace; 3 — water cooling pipe; 4 ~ stainless steel pressure chamber; 5 — Si wafers; 6 — susceptor; 7 — susceptor push r o d ; 8 — cap push r o d ; 9 — exhaust line; 10 — cold t r a p ; c — d.c. or R F plasma-enhanced anodic oxidation of GaAs (after Sugano [162]; reprinted with permission from T H I N SOLID F I L M S , Copyright (g) 1980, Elsevier Sequoia S.A.): 1 — quartz chamber consisting of a bell j a r ; 2 — d.c. anodisation source; 3 — high-frequency oscillator; 4 — cathode; 5 — anode; 6 — quartz shields; 7 — GaAs sample; 8 — heater; 9 — thermocouple; 10 — 0 2 gas inlet; 11 — needle valves; 12 — rotary p u m p ; 13 — Pirani gauge; d — laser-enhanced oxidation of Si in dry 0 2 : 1 — argon or carbon dioxide laser; 2 — power meter; 3 — beam scanner; 4 — lens; 5 — growth cell; 6 — quartz window; 7 — wafer pedestal; 5 — resistance h e a t e r ; 9 — temperature measurement and control; 10 — wafer; 11 — oxide l a y e r ; 12 — oxidant gas (dry oxygen or steam) inlet; 13 — exhaust.

41

FUNDAMENTALS

2.7.2 Plasma Conversion of Substrate Plasma anodisation [156—171] consists of forming a thin oxide film by placing the substrate under bias in the field of an oxygen or nitrogen plasma. Plasma anodisation is used for the oxidation [156—163] and nitridation [164-171] of elemental [156-161, 164-169] and compound [162, 163, 170, 171] semiconductors. There are two main advantages of this process over thermal surface conversion, namely low substrate temperatures and fast oxidation rates. Low substrate temperature (~300°C for GaAs, ^600°C for silicon) prevents decomposition or evaporation of some constituents of compound semiconductors and removes the generations of oxidation-induced stacking faults and the redistribution of impurities in the silicon substrate. The equipment used in anodisation is shown in Fig. 2.6c. 2.7.3 Laser Conversion of Substrate Substrate surface conversion (oxidation or nitridation) can also be obtained applying photochemical methods by using ultraviolet radiation or laser excitation in the presence of an oxygen- or nitrogen-containing molecule [172—179]. This approach, investigated recently for the purpose of silicon and GaAs passivation, offers several advantages, such as low sample temperature (as opposed to thermal oxidation), clean and dry ambient (as opposed to anodic oxidation), and the capability of producing localized film growth. However, the oxidation rate is much lower and the equipment (Fig. 2.6d) is more complicated. Photo-enhanced oxidation has been reported for Si [172-177], GaAs [178], ZnTe, ZnSe, and (HgCd)Te using UV light as well as visible or infrared laser irradiation of heated wafers in the presence of 0 2 , CO, NO, or H 2 0 molecules. Laser nitridation of Si has also been reported [179].

2.8 Chemical Vapour Deposition Chemical vapour deposition (CVD), as its name implies, means the formation of a stable film on a substrate, produced by the reaction of chemicals from the gaseous state making use of an activation energy. The equipment for thin film deposition using CVD generally contains the following units: gas or volatile liquid sources; a gas distribution and mixing system; a reaction chamber; a system for providing the activation energy for the reaction and for heating the substrates; and a neutralization system for the exhaust gases (Fig. 2.7a). The equipment design depends on the type of activation energy, the initial aggregation state of the sources, the reactor operating principle, the substrate heating type, the reactor configuration, and the wall temperature. In general, the variables affecting the deposition rate and film properties are the nature of the reactants and their purity, the amount of energy supplied, 42

TECHNIQUES OF P R E P A R I N G T H I N FILMS

RESISTANCE INDUCTIVE ' OR RADIANT HEATING

1 {CARRIER GAS [PURIFIER

GAS SUPPLY

GAS FLOW CONTROL —w \REACTOR\

•^

SCRUBBER

t

TEMPERATURE MEASUREMENT AND CONTROL .Fig. 2.7 Block diagram of set-up for atmospheric pressure CVD.

the substrate temperature, the ratio of reactants, the gas flow rates, the system pressure, the geometry of the deposition chamber, and the substrate surface preparation. The main advantages of this technique consist in producing uniform, reproducible and adherent layers of all classes of materials without defects and impurities at relatively high rates in simple and cheap commercially available non-vacuum equipment. However, there are some disadvantages, among which are the use of comparatively high temperatures in many processes and chemical hazards caused by toxic, explosive, inflammable or corrosive gases. CVD processes can be classified according to the type of their activation energy, namely thermally-activated CVD, plasmaenhanced CVD, photochemical CVD, laser-induced CVD and electron-beam assisted CVD.

2.8.1 Thermally-Activated CVD Thermally-activated CVD uses thermal energy to produce a gas-phase chemical reaction resulting in the formation of a thin film on a substrate. Depending on the pressure value in the reaction environment, there are two main classes of thermal CVD: atmospheric-pressure CVD and low-pressure CVD. Both atmospheric- and low-pressure CVD can be subdivided, in turn, into high-temperature and low-temperature CVD, if the substrate temperature is higher or lower than 500°C, respectively. If at least one of the reactants is a metallo-organic compound, the techniques are usually called metallo-organic CVD (MOCVD) or low-pressure metallo-organic CVD (LPMOCVD), as proposed by Manasevit. An alternative nomenclature, namely organometallic CVD (OMCVD) and low-pressure organometallic CVD (LPOMCVD) can also be used. However, it is not preferred, although the term organometallic is rigorously more correct. I t may be considered as a special type of CVD process, a process in which the substrate also participates in the film forming reaction as one of the reactants, i.e. substrate thermal conversion b y means of chemical vapour. 43

FUNDAMENTALS

In fact, reaction with the substrate is not a deposition, but rather a growth process. This method has therefore been treated separately. By adopting an extended definition of CVD, we can include pyrolytic spray processes. Indeed, spray pyrolysis resorts to a fine spray of a suitable solution on a hot substrate in an open area in order to achieve the desired coating. If the coating is obtained from a heterogeneous (at the substrate surface) or homogeneous (in the gas phase) reaction of liquid droplets, this process is not a CVD process. Spray pyrolysis is a true CVD process only when the precursors are reactant vapours as is the case when the droplets are completely vaporized and a coating results from heterogeneous or homogeneous reactions. 2.8.1.1 Normal-Pressure Thermally-Activated CVD

In the atmospheric-pressure thermally-activated CVD method gaseous reactants (or vapours obtained from the liquid or solid reagents) are diluted by an inert gas (H2, N2, argon, or helium) and flow over substrates maintained at high temperatures in a reactor where the total pressure is 1 atm. High-temperature CVD. High-temperature CVD (HTCVD) proceeds under normal pressure conditions and at temperatures higher than 500°C. In general, temperature increase results in an enhanced film deposition rate, an improved crystalline lattice, greater density, promotion of certain otherwise impossible reactions, and the ability to perform in-situ substrate gas etching. This method is useful only for substrates able to withstand heat treatment in the gaseous ambient. In addition, the properties of some important films, such as Si0 2 , are not equivalent to those prepared using thermal oxidation, even though the films are annealed at high temperatures. Also, high-temperature deposition produces stress, interdiffusion or melting of device structures, as well as degradation of the substrate. HTCVD is the most versatile and widely used technique for large scale preparation of most semiconducting, insulating, superconducting and magnetic films, in- epitaxial, polycrystalline or amorphous form, usually starting from halide reactants. High-temperature reactors can be RF induction-, resistance-, or radiant-heated and usually have a horizontal, barrel, or vertical configuration (see Chap. 11). Thin films obtained by using HTCVD are summarized in Table 2.1. HTCVD equipment is presented in Fig. 11.3. In some situations HTCVD can be performed advantageously in the presence of acoustic wave (ultrasonic) irradiation this variant being called acoustic wave CVD (AWCVD) [180, 181]. Low-temperature CVD. Low-temperature CVD (LTCVD) occurs under normal ρΓβ58μΓ6 conditions and at temperatures below and up to 500°C [182]. In general, temperature decrease is imposed by some substrates and/or by device structures incorporating some type of metallization layer, e.g. a gold or an aluminium metallization layer permitting temperatures up to 330°C or 475°C, respectively. This technique is used for depositing insulating films, especially oxide and silicate glass films, based on hydride oxidation reactions. LTCVD is usually performed in resistance-heated hot-wall reactors of three main types: horizontal, vertical, or continuous (disperser-, nozzle-, or 44

TECHNIQUES OF PREPARING THIN FILMS TABLE 2.1 Present Status of Thin Films Achievable by Using CVD Techniques

3 4 5 6 12 13 14

15 21 22 23 24 25 26 27 28 29 30 31

32 33 39 40 41 42 44 45 46 47 48 49 50 51 57 58 60 62 63 64

65 66 61 68 69

70 71

z

CVD films

0

1 LiNb03 Be, BeC, Be 3 N 2 , BeO, BeSi B, B4C, BN, B 2 0 3 , ΒΟ^, ΒΡ, BAs C, {0χΉ.ν)η Mg, MgO, MgFe 2 0 4 AI, AlB, AIN, A1 2 0 3 , A l 2 0 3 - C r 2 0 3 , AIP, AlAs, AlSb, (AlGa)P, (AlGa)As, (AlGa)Sb, (Alln)P Si, SiB, SiC, Si0 2 , BSG, PSG, AISG, AsSG, LSG, ZSG, BPSG, GBSG, A1PSG, A1BSG, LBSG, ZBSG, SiO^Nj,, SiO^NyH^ Si 3 N 4 , S i N , H y , SiGe P 2 0 5 , P3N5 ScN, ScP, ScAs, Sc(AsP) Ti, TiB 2 , TiC, TiCN, TiN, TiO a , TiSi, TiTa V, VB 2 , VC, VN, V 2 0 3 , V 0 2 , V 2 0 5 , VSi Cr, CrB2, Cr0 2 , Cr 2 0 3 , CrAl, CrSi, CrS, CrSe, CrTe Mn, Mn0 2 , MnSi, MnFe 2 0 4 Fe, FeB, FeO, Fe 2 0 3 , F e 3 0 4 , FeFe 2 0 4 > FeSi, FeSn Co, CoO, Co 3 0 4 , CoSi, CoFe 2 0 4 Ni, NiB, NiO, NiSi, NiCr, NiFe Cu, CuB, CuO, CuInS 2 , CuInSe 2 Zn, ZnO, ZnS, ZnSe, ZnTe, Zn(SSe), (ZnCd)S, ZnSiP 2 , Zn 2 Si0 4 : Mn ZnSiAs 2 , ZnGeAs 2 , ZnCr2S4, Z n S - G a P , Z n S e - G a P , Z n S e - G a A s Ga, GaN, GaP, GaAs, GaSb, Ga(AsP), Ga(AsSb), (Galn)P, (Galn)As, (Galn)Sb, (GaIn)(AsP) Ge, GeC, Ge 3 N 4 , Ge0 2 As YN, Y 3 Fe 5 0 1 2 , Y 3 Fe 5 _ a; Ga a: 0 12 Zr, ZrB 2 , ZrC, ZrN, Zr0 2 , ZrSi Nb NbB, NbN, Nb 4 N 5 , NbC, Nb 2 0 5 , NbSi, Nb 3 Si, Nb 3 Ga, Nb 3 Ge, Nb 3 Sn Mo MoB 2 , Mo2C, MoSi, MoSi 2 , MoW Ru RuOo Rh Pd, PdSi Ag, AgSi Cd, CdO, CdS, CdSe, CdTe, Cd(SSe), C d S - I n P , CdCr2S4> Cd 2 Sn0 4 , (CdHg)Te In,' InP, InAs, InSb, In(AsP), In(AsSb), l n 2 0 3 Sn, SnO a , SnO a : Sb, SnS, SnSe, SnTe, (SnPb)Se, (SnPb)Te Sb LaN Ce, Ce 2 0 3 NdgFeg.^Ga^O^ Sm 3 Fe 5 _ a; Ga a; 0 12 Ga-O '3^12 -x Gd 3 Fe 5 0 1 2 , G d g F e s ^ G a ^ a Tb 3 Fe 5 0 1 2 , T b g F e ^ G a ^ a DyN, DygFeg^Ga^O^ Ho 3 Fe 5 0 1 2 , HogFeg^Ga^Oia ErN, Er 3 Fe 6 0 1 2 , E r g F e ^ G a ^ a Tm 3 Fe 5 0 1 2 , Tm3Fe5_a;Gaa;01a YbN, Yb 3 Fe 5 O i a , YbgFeg-^Ga^Oia LuN, Lu 3 Fe 5 _a;Gaa;012

45

FO N D AMENTALS

TABLE 2.1 (continued) 0

1

72 73

Hf, HfB 2 , HfC, HfN, H f 0 2 , HfSi Ta, T a B 2 , TaC, Ta 2 C, TaN, Ta 2 N, Ta 3 N 5 , Ta 2 O s , TaAl, TaAIN, TaSi TaNb, T a W TaTi W, W B , WC, W 2 C, W 0 3 , WSi 2 , W 5 Si 3 , WMo, WMoRe, W R e Re Os Ir Pt, PtSi Au, AuSi, HgTe Tl P b , P b O , P b 0 2 , P b T i 0 3 , PbFe 1 2 0 1 9 , P b S , PbSe, PbTe, Pb(SSe) Bi Th, Th 3 N 4 , T h 0 2 , ThSi

74 15 76' 77 78 79 80 81 82 83 90

Note: The materials are ordered by increasing atomic number (Z) of the single elements, or of the first element of compounds or alloys.

injector-type) reactors (see Chap. 11). The main parts of LTCVD equipment are shown in Fig. 11.2. MOCVD. MOCVD is based on the decomposition, usually at normal pressure, of an organometallic compound in a flow of carrier gas, mainly with the use of thermal energy, resulting in the formation of a thin film [183, 184, 2595]. This technique has advantages compared with both non-CVD and CVD techniques of film formation. By employing all starting materials in the vapour state in a simple cold-wall reactor having only one heated temperature zone, this technique allows the economic and highly productive deposition of uniform and adhesive films at low substrate temperatures, as well as the elimination of autodoping and impurity incoporation from the reactor walls. Disadvantages include unavailability of high purity reactants, difficult handling of toxic, volatile and often pyrophoric OM reactants, and non-equilibrium deposition processes. MOCVD enables semiconductor, insulator, conductive and resistive layers to be prepared in amorphous, poly crystalline or single crystalline forms, as used in electronic, optoelectronic, microwave and solar energy devices. The main factors determining the film deposition rate and properties are the nature of the OM reagent, the temperatures of the substrate and the evaporator, the rate of OM transport, and the impurities introduced into the system. The apparatus is very similar to that used in LTCVD, containing in addition an evaporator in which the initial liquid or solid OM is evaporated (Fig. 2.8). Spray pyrolysis. This method is based on the pyrolysis of a fine mist of an organic or aqueous solution of one or more metal salts on a heated substrate, on which the reaction is also produced [185—203]. Spray pyrolysis (hydrolysis) was mainly used to deposit some wide band gap semiconductors (metallic oxides) which have a large range of application in solar energy on glass substrates. A complete list of pyrolytic spray coatings is given in Table 2.2. The main process control parameters in spray pyrolysis are the starting 46

TECHNIQUES OF P R E P A R I N G T H I N FILMS

Fig. 2.8 Apparatus for OMCVD (rotating planetary reactor), employing solid source materials (used, for example, to deposit SnO a films) (after Vossen [4638]; reproduced by permission of Academic Press, Inc.): 1 — (argon) carrier gas; 2 — furnace ; 3 — source ; 4 — Arheating t y p e ; 5 — secondary reactant (oxygen); 6 — substrates; 7 — heater.

reagent composition, the substrate temperature, the angle of incidence of the spray to the substrate, the amount of water in the starting reagents, the ambient humidity, the impurity concentration and type, and the flow rate of the gas and solution. The diagram of typical apparatus for spray pyrolysis is given in Fig. 2.9. Spray pyrolysis, applied usually to metal chloride hydrolysis, is a high-efficiency process for depositing uniform films with suitable electrical or optical properties on large area substrates. However, the method has several disadvantages, namely the deleterious effect of atmospheric impurities, the waste of starting reagents, the introduction of impurities from the glass substrate which is attacked by HC1 liberated in the reaction, and substrate cooling by the spray. Although new improved CVD variants (LPCVD, PECVD, MOCVD, UVCVD, LCVD, etc.) have been developed more recently, the conventional APCVD (HTCVD and LTCVD) has remained, with few exceptions, the main technique used in both developmental and industrial applications. APCVD is presently used successfully in the fabrication of all types of electronic and optoelectronic devices ranging from discrete mesa-type diodes to complex integrated circuits. The main application of MOCVD at the present time is to the realization of AlzGa^As/GaAs heterojunction devices such as lasers, LEDs, solar cells, FETs, photocathodes, photodetectors, and bioplar he tero junction transistors.

· ·

· ·

· ©

· ·

—Ezäzszr— \l//

I

«

Fig. 2.9 Apparatus for spray hydrolysis (after Vossen [4638]); reproduced by permission of Academic Press, Inc.): 1 — high pressure air or 0 2 ; 2 — pressure regulator; 3 — heaters; 4 — solution being sprayed (e.g. SnCl 4 ); 5 — metering valve; 6 — substrate.

47

FUNDAMENTALS

TABLE 2.2. Examples of Coatings Deposited by Spray Pyrolysis [185, 186, 5217] Coating material

No.

Substrate

1 2

ZnO CdS:In

glass glass

3

CuInSe 2

glass

4

SnO a

glass n —Si glass n-Si glass Si glass glass glass glass glass

5 6 7 8 9 10 11 12 13

ln203 In 2 0 3 :Sn(ITO) Cd 2 Sn0 4 Ti02 Fe203 Cr 2 0 3

v2o3 Pd Ru

Reactants ZnCl 2 CdCl 2 + thiourea+InClg CuCl + InCl 2 + dimethylselenourea SnCl 4 SnCl 4 In(acac) 3 SnCl 4 + InCl 3 CdCl 2 + SnCl 4 Ti(OC 2 H 5 ) 4 Fe(acac) 3 Cr(acac) 3 V(acac) 3 Pd(acac) 3 Ru(acac) 3

Solvent H20 H20

Substrate temperature (°C)

Refs.

4 0 0 - 5 0 0 [188, 189]

H20

325 260

[191-194] [196, 198]

H 2 0 + HC1 ethylacetate acetylacetone H20 H20 w-butylacetate butanol butanul butanol butanol butanol

400-500 300-400 470-520 400-500 370-450 200-450 400-550 520-560 450-510 300-350 380-400

[185] [200] [185] [201] [187, 202] [203] [185] [185] [185] [185] [185]

Other III—V semiconducting compounds, such as InP, Ga^In^As and Ga1^Ina;Pi/As1_2/, have also been developed extensively for their use in FETs, lasers, and solar cells. MOCVD is also used in industrial production of photodetectors based on II—VI semiconducting compounds such as Cd^Hg^Te. MOCVD is capable to realize synthetic heterostructures containing either ultrathin layers or atomically abrupt interfaces which are useful in special devices. These include single quantum-well and multiple quantum-well heterostructures for lasers, doping superlattices, modulation-doped superlattices, strained-layer superlattices, and heterojunction superlattices for high-frequency oscillators and two-dimensional electron gas structures for FETs. Also, MOCVD has become a production technique for the fabrication of FETs, laser diodes, and photocathodes [2595]. SP has been used successfully in production for a variety of photovoltaic, solar-collector and glazed-window applications. 2.8.1.2 Low-Pressure CVD

LPCVD is a process of thin film deposition on heated substrates at high or low temperatures in a reactor under reduced pressure, usually ^ 1 torr (0.01 — 100 torr [204—206]. LPCVD has the following advantages: suppression of autodoping from the substrate and vapour phase, improvement in film thickness and composition uniformity, control of the deposition rate only by the surface reaction rate, decrease in defect number, improvement in step coverage suitability for large-scale production. Its main disadvantages are the lower deposition rates and the increased cost and maintenance. 48

TECHNIQUES OF PREPARING THIN FILMS

\CARRIER GAS\ 1 PURIFIER 1 1 CONTROL 1 1 UNIT \

~TZ

\RESISTANCE \0R RADIANT

j HEATING

ii ^^

GAS SUPPLY 1

I WENT

I

^-~GAS FLOW 1 w CONTROL 1

1 *

^

-^

REACTOR ■ 1

^W

|sc/?(yae£/? |

PUMPING 1 UNIT

Fig. 2.10 Basic components of set-up for low-pressure CVD (after Krullmann and Engl [2385]; reprinted with permission from IEEE TRANSACTIONS ON ELECTRON DEVICES, © 1981 IEEE).

Low pressure CVD systems (Fig. 2.10) include mainly a resistance-heated horizontal tube or a cylindrical geometry RF-heated reactor (see Chap. 11). Film deposition rate and properties for a given reactant system are influenced by the substrate temperature, the total pressure of the gases present, the partial pressure of the reactant species, and the background pressure and pumping rate of the vacuum system. For a number of important applications in solid-state technology, such as the production of insulator and semiconductor films for the actual generation of integrated circuits (very large scale integration devices), this technique has replaced conventional APCVD. Thin films prepared using LPCVD are summarized in Table 3.2. LPMOCVD. LPMOCVD is a combination of LPCVD with MOCVD methods. This technique was originally developed for the growth of submicron monocrystalline silicon, gallium arsenide and other related III—V compounds for the application of both microwave and optoelectronic devices. Presently LPMOCVD has been extended to other classes of compounds, such as metals or insulators. Compared with conventional CVD or MOCVD, this technique offers the following advantages: reduction of autodoping, elimination of undesired secondary reactions, improvement in film uniformity due to operation at higher gas velocity in the diffusional growth regime, the possibility of growth on large areas of semiconductor substrates, and improved conformal coverage. LPMOCVD equipment consists of either a conventional horizontal RF-heated reactor or a diffusion-type hot-wall tube furnace, and an associated gas distribution scheme including MO evaporators (Fig. 11.4). LPMOCVD films are also included in Table 2.3. 2.8.2 Plasma-Enhanced CVD In this method, a glow discharge is produced in the gaseous reactant mixture which is maintained at a pressure of 0.1 — 1 torr under an RF plasma [207—221]. The specific complex reactions which take place between the dif49

FUNDAMENTALS

ferent species existing in the glow discharge plasma as well as the interaction between the plasma and the substrate, lead to the formation of films on various wafers which are maintained in a wide temperature range from 25°C to the higher temperatures used in conventional CVD processes. The advantages of the method are the capability of producing conformal films at comparatively low temperatures (typically 200—400°C), relative insensitivity to wafer temperature, and the possibility of obtaining films with amorphous structure on various heat-sensitive substrates. Its main disadvantages are low deposition rate and efficiency, difficult control of film composition and thickness uniformity, inability to handle solid or liquid react ants, nonstoichiometric and inhomogeneous films, exposure of substrate and film to radiation damage, complicated and expensive equipment, and limited use on a production scale. The film deposition rate depends on the R F energy, the mole fraction of the reactants, the total pressure, the substrate temperature, the reaction geometry and the nature of the substrate. A PECVD system contains the following components: a deposition chamber; an R F generator provided (eventually) with an impedance matching network; a gas control panel; and a vacuum pump with a pressure measuring device (Fig. 2.11). PECVD is usually performed in a vertical reaction chamber or in a bell jar chamber (the diode system), the glow discharge plasma being excited either inductively from the outside of the reaction chamber, or capacitively within the deposition chamber, respectively. Schematic diagrams of the three main types of R F glow discharge reactors (the inductively-coupled vertical-tube reactor, the inductively-coupled vertical parallel-plate reactor, and the capacitively-coupled horizontal parallelplate reactor) are given in Chap. 11. Films deposited to date using PECVD are summarized in Table 3.3. PEMOCVD. PEMOCVD is a plasma-promoted CVD process using MO reagents. This technique has recently attracted considerable attention, as a means to obtain amorphous films of semiconductor compounds which are of potential interest for electronic or optoelectronic devices. It can also be used to prepare metal oxides or metal films. This technique allows cheap deposition of amorphous semiconductor compounds, starting from commercially availI GAS STREAM [PREPARATION ii

RF POWER SUPPLY AND MATCHING NETWORK

yf 1'

DEPOSITION ^^ CHAMBER

"*~Ί

1 MEASUREMENT AND CONTROL] 1 OF PROCESS PARAMETERS

[SUBSTRATE \ HEATING VACUUM SYSTEM EXHAUST HAND

1 REACTANTS AND\ [CARRIER GASES \

Fig. 2.11 Block diagram of equipment for PECVD.

50

TECHNIQUES OF P R E P A R I N G T H I N F I L M S

able liquid organometallics at room temperature, at a convenient growth rate, on heat-sensitive glassy, metallic, or polymeric substrates. However, there is an increased probability of film contamination by side-products of OM plasma decomposition, especially carbon or hydrogen. Typical PEMOCVD systems are given in Fig. 11.5, whereas the list of PEMOCVD films is included in Table 3.3. PECVD is extensively used in production applications -to discrete and integrated devices using Si3N4 films and photovoltaic cells based on a-Si films. 2.8.3 UV Radiation-Enhanced CVD Photochemical-assisted vapour deposition (photo-CVD) is based on the utilisation of ultraviolet light to promote the decomposition of reactant gases at low pressure (0.3—1 torr) [222]. There are two basic photo-CVD processes: Hg-sensitized photolysis and direct photolysis. Hg-sensitized photolysis uses mercury vapour for absorbing UV radiation at a wavelength of 2537 Ä, followed by catalytical transfer of the energy to one (or both) of the reactant gases, thus enhancing its chemical reactivity. Direct photolysis is based on UV excitation of reactant gases without mercury gas sensitizing. Both processes use the same photochemical reactor system, formed of the following main parts: a reaction chamber having heated substrates illuminated by an external UV lamp, a gas flow control scheme which can include an Hg reservoir, a chemical-resistant pump, and an exhaust scrubber (Fig. 2.12). The deposition rate and film properties depends primarily on intensity, reaction chamber geometry, chamber pressure, pumping speed, and only weakly on substrate temperature. The main advantages of this technique are the very low temperature (50—200°C), the avoidance of radiation device degradation, the minimization of the occurrence of typical high-temperature negative effects in semiconductor processing (such as wafer warpage, defect

I I

I I

1 Ϊ 1

]NH3

\N02

\siHA

]N:

Pig. 2.12 UV radiation-enhanced CVD system (photo-CVD of SiO a and Si 3 N 4 ): 1 — gases; 2 — filters; 3 — purge valve; 4 — flowmeters; 5 — Hg reservoir; 6 — mercury vaporizer; 7 — vent valve; 8 — reaction chamber ;9 — substrate; JO — substrate heater (hot plate or I R lamps); 11 — transparent window; 12 — UV l a m p ; 13 — throtle valve; 14 — t r a p ; 15 — chemical p u m p ; 16 — exhaust diluent valve; 17 — exhaust scrubber. {

51

FUNDAMENTALS

generation, dopant diffusion, Al hillock formation, Al/Si interface deterioration), excellent conformal coverage, the preparation of film with no thermally induced mechanical stress, and the processing on some temperature sensitive compound semiconductors (InP, etc.). The major disadvantages of this technique are the relatively low deposition rate (which in direct photolysis is lower than in the Hg-sensitized process by about an order of magnitude), potential Hg contamination,nonuniformity among wafers, and low throughput. This technique was applied to the deposition of amorphous silicon and insulating films (Si0 2 , Si3N4, Si^O^Ng and phosphosilicate glass) for devices such as solar cells and MOS. Photochemical-assisted MOCVD. Metals of interest to the electronic device can be deposited conveniently by using vapour-phase ultraviolet dissociation of an organometallic source (e.g. a metal alkyl or carbonyl). The process is usually accomplished by using direct photolysis, irradiating either with ultraviolet (UV), or vacuum ultraviolet (VUV) lamps. In the latter case, the VUV-CVD system includes a microwave-excited rare gas VUV lamp, a vacuum chamber, a substrate, and an organometallic source of photo-active vapour. Thin films, obtained to date, by UV radiation-enhanced CVD are presented in Table 3.4.

2.8.4 Laser-Induced CVD Laser-induced CVD (LCVD) can be achieved using either pyrolytic or photolytic decomposition of gaseous phase molecules [232—235]. Pyrolytic LCVD is based on local substrate heating by means of I R or visible laser light, which is not absorbed by gaseous phase molecules. Photolytic LCVD is based on electronic or vibrational excitation of the gaseous molecules by using a UV or I R laser. LCVD allows high-rate one-step local deposition of insulating, semiconducting, and metallic materials for the production of microstructures. The deposition rate and physico-chemical properties of the layer obtained depend on parameters such as laser irradiance, focus diameter and scanning velocity, as well as substrate local temperature and system gas pressure. A typical experimental set-up for LCVD contains an I R visible or UV laser, a means for expanding and scanning the laser beam, a pyrometer for measuring local substrate temperature, a microscope for film thickness and deposition rate measurement, and a reactor containing the movable substrate which is connected to a gas supply (Fig. 2.13). Laser-induced MOCVD. Laser-induced gas-phase photolysis as well as pyrolysis of metal alkyls or hexacarbonyls have been used for single-step

Gas

52

Fig. 2.13 Typical experimental set-up for laser CVD: 1 —laser; 2—variable a t t e n u a t o r ; 3 — power meter; 4 — lens; 5 — reactor; 6—transparent window; 7—pyrometer; 8 — substrate; 9 — localized deposited film; 10 — gas supply; 11— gas outlet.

TECHNIQUES OF P R E P A R I N G T H I N F I L M S

formation of patterned metal deposits on various semiconducting or insulating substrates. Compared with the electron-beam technique, LMOCVD enables a reduction of film contamination, a decrease in the number of local film defects, and processing in a flow of reactant and inert gases. The disadvantages of this technique are the relatively short lifetime of continuous lasers, and the limited variation of process parameters. Thin films obtained using laser-induced reactions of both inorganic or metallo-organic compounds are listed in Table 3.5. LCVD is potentially very useful in the fabrication of integrated circuits by direct deposition of dielectric and metal patterns. 2.8.5 Electron-Beam Assisted CVD This deposition technique uses an electron beam to generate a spatially confined plasma reaction in a small volume, deposition occuring on a heated substrate (150—500°C) located directly beneath that region [223—231]. Electron-beam assisted CVD allows relatively high rate deposition of some dielectric films (Si0 2 , Si3N4) at low (200—350°C) substrate temperatures with a conformal coverage of uneven surfaces such as Al and poly-Si steps. The reactants used are the same as in conventional APCVD (SiH4, N 2 0 and NH3), the total chamber pressure being 0.1 — 1 torr. A typical experimental arrangement for electron beam-assisted CVD is depicted in Fig. 2.14. Thin films obtained using electron-beam assisted CVD starting from both inorganic and metallo-organic reactants are presented in Table 3.6. Electron-beam assisted MOCVD. OM decomposition by electron beams has been used for depositing films with a certain pattern. The electron beam is directed to the substrate which is surrounded by OM vapour; under the influence of electrons, decomposition of OM vapour near the substrate occurs leading to film formation. The main advantage of this technique is the achievement of high accuracy low-size microcircuils without needing masks. Limitations consist of film contamination by secondary nonvolatile products of OM decomposition reactions, process disturbances caused by film deposition on various parts of the electrono-optic system and the potential for semiconductor device degradation. EBCVD has so far been used in multilayer devices to deposit dielectric layers selectively and conformally [225].

LVi] ^. Fig. 2.14 Apparatus for electron-beam CVD: 1 «■* electron-beam source ; 2—system for magnet and electric focusing and deflection of the electron b e a m ; 3 — substrate ; 4 — heater; 5 — OM or inorganic reactant vapours; 6 — deposition chamber; 7 — vacuum pump.

2

.6

I

ΡΤΊΓΙ

*' 53

FUNDAMENTALS

2.8.6 Ion-Beam Assisted CVD (IBCVD) This version of CVD uses a focused beam of ions (e.g. Ga+) to induce deposition from a suitable gaseous ambiant (Fig. 2.15). Similarly to LCVD and EBCVD, IBCVD can be used as a nonlithographic technique for producing patterned material with submicron resolution determined by the beam diameter. So far, deposition of Al, Au, W, and C from A1(CH3)3, dimethyl gold hexafluoroacetylacetonate (C7H7F602—Au), WF 6 , and hydrocarbons, respectively, have been reported. IBCVD is a potentially high resolution (0.05 μιη) deposition technique, but the deposits obtained at present contain a high percentage of impurities -either C or O [236, 237]. The main application of IBCVD at the present time is the repair of defects in photomasks by substituting deposited carbon for missing chrome [236].

2.9 Comparison between CVD and Other Thin Film Deposition Techniques The CVD and non-CVD techniques used for achieving thin films in solid-state technology are compared in Table 2.3 taking into account aspects such as source materials, deposition parameters, film structure and composition, and typical films and applications. Each film preparation technique has its own advantages and disadvantages. Therefore, the choice of a method depends on the specific application, namely requirements for film properties, temperature limitations of the substrate, and compatibility of the process with preceding and subsequent processing steps. However, among the numerous methods described earlier, only vapour deposition lends itself to the need for the miniaturization of today's electronics technology. Indeed, in electronics thin film materials have been almost exclusively prepared by using physical and chemical vapour deposition methods. These two main preparation techniques, PVD (evaporation, sputtering, MBE, etc.) and CVD, although they have some limitations, are both competitive and complementary. Within the past 25 years, CVD has become the main technological method for producing films for semiconductor devices, the older PVD playing only a secondary role. Nevertheless, PVD methods, as well as other methods (LPE, etc.) are being continuously improved to extend their use to new materials, processes and technologies encountered in solid-state electronics. '

»

32\L

"L :©=

f N

2

54

5

^4

Fig. 2.15 Schematic diagram of IBCVD system (after Shedd et al. [236]; reproduced by permission of The American Institute of Physics): 1— focused ion beam (Ga+) source; 2—substrate; 3 — X — Y stage; 4 — reactant source; 5 — reactant feed tubing; 6 — vacuum enclosure (P = 1 — —4 x 10~4 ΤΌΓΓ) ; 7 — vacuum pumps.

1

No.

0

Electroless plating

Electrolytic anodisation

2

3

5

Substrate -f- electrolyte

Solution of metallic salt -f various additives

Electrolyte

Ion-beam deposition

evaporation

j Vacuum

Metals or semiconductors

Metals or semiconductors (Si, GaAs)

Metals

Solid target

3

Substrates

Any

High-vapour Any pressure solid

Vacuum envircmment

Electroplating

1

4

1

2

Source material

Electrolytic en vironment

Deposition process

1

1-2

Non-linear 0.010.1 μιη in 1 min.

~0.1

0.01-100

5

Typical deposition rate (μηι/min)

25-750 j 0.01-0.1

25-600

25-70

25-70

25-70

4

(°C)

Typical substrate temperature

6

Amorphous, polycrystalline, or single-crystalline

Amorphous, polycrystalline, or monocrystalline

Amorphous

Polycrystalline

Polycrystalline

1

Crystalline nature of deposit

Comparison of the Main Techniques Used for Thin Film Formation [99, 100]

TABLE 2.3

Ion beam target

Filament/ crucible, reactor walls

Electrolyte

Components of t h e plating b a t h

Electrolyte

7

Sources of impurities

Si, GaAs, Si 3 N 4

Al, Au, Ni-Cr

A1 2 0 3 , Ta 2 O s , Si02

Ni, etc.

Cu, Ni, etc.

8

Typical films formed

Superconducting layers

Metallization or resistive layers

Passivation layers

Conducting layers

Conducting layers

9

Typical electronics applications

[120-127]

[110]

[108, 109]

[104-107]

[103]

10

References

en

10

9

Ion plating

2

Substrate -f ion source

Any

High-vapour A n y pressure solid

Low-vapour pressure solid

Any

High-vapour Any pressure solids

High-vapour Any pressure solids

environment

Sputter deposition

Plasma

Ion-implantation

Hot-wall epitaxy

7

8

Molecularbeam epitaxy

1

6

0

TABLE 2.3 (continued) 3

5

25-200

25-250

25-400

0.01-1

0.001-0.01

—

250-500 0.01-1

5 5 0 - 1 0 5 0 0.01

4

Amorphous, polycrystalline

Amorphous, polycristalline

Amorphous

Singlecrystalline

Singlecrystalline

6

Crucible, reactor walls, solid source, reactive gases

Solid sources, reactor walls, reactive gases

Substrate surface

Some solids, reactor walls

Effusion cells

7

Si 3 N 4

W, Mo, WSi 2 , MoSi2, Si 3 N 4

Si 3 N 4 , SiO a ,SiC

III-V, II-VI, IV-VI semiconductors

GaAs, A l j^ G a^ A s

8

[135-139]

[134]

[128-131]

10

Dielectric materials for some SDs

[140-142]

Conducting 1 [111-119] and insulating layers for some SDs and ICs; superconducting layers

Dielectric l a y e r s for some SDs

Epitaxial layers of semiconductors for optoelectronic devices

Epitaxial layers of semiconductors for microwave and optoelectronic devices

9

1 Supersaturated solution

environmen t Any oxidizable substrate

Any oxidizable substrate Any oxidizable [ substrate

Substrate + gas

Substrate + gas

Substrate + gas

Plasma conversion (oxidation)

Laser conversion | (oxidation)

14

15

Any

-

Singlecrystalline

1 Singlecrystalline

25-250

300-600

Non-linear

Non-linear

Substrate surface

Substrate surface, reactor walls

Substrate surface

Solution materials, gaseous ambient

Amorphous, Substrate polycrystal- j surface line

Amorphous, polycrystalline

Amorphous, 8 0 0 - 1 2 0 0 Non-linear 1 μπι in 1 h polycrystalline

400-600

Any I 600-10001 0 . 1 - 1 substrate withstanding growth temperature

Thermal conversion (oxidation)

a. Converted s ubstrate

Chemical vapo ur

Solid

environm ent

Solid-phase epitaxy

Solid

Liquidphase epitaxy

environment

13

12

11

Liquid

Si0 2 , GaAs 1 oxide

Si02, Si 3 N 4 , III-V oxides

Si0 2 , Si 3 N 4

Si

AlyGa^^As R 3 Fe 5 0 1 2

[147, 148]

[143-146]

[156-171]

Passivation [ 1 7 2 - 1 7 9 ] layers in SD i technology

Passivation layers in solid-state technology

Passivation [ 1 4 9 - 1 5 5 ] layers, diffusion masks, etch masks in fabrication of, numerous SDs a n d ICs

Potentially useful for obtaining epitaxial Si

Semiconductors used in microwave field and optoelectronics ; magnetic materials

g

2

Atmospheric pressure CVD

Low-pressure CVD

Plasmaenhanced CVD

UV-radiation enhanced CVD

17

18

19

3

1

4

Gases, volatile liquids

Gases, volatile liquids

Gases, volatile liquids

Any

Any

0.1-1

0.01-0.1

0.1-1

5

5 0 - 2 0 0 0.005-0.015

25-250

Any sub500-1200 strate with- 2 5 0 - 5 0 0 standing growth temperature

1 500-120 Gases, vola- I Any subtile liquids, strate with- 2 5 0 - 5 0 0 high-vapour standing growth pressure temperature solids

b. Unmodified 1 substrate

1

16

0

TABLE 2.3 (continued) |

8

Gas source, Epi-Si, reactor epi- GaAs walls, sus- G a A s ^ P t f , ceptor poly-Si, SiO a , Si304, Nb 3 Sn

7

Amorphous, polycrystalline, single-crystalline

Amorphous, poly crystalline, epitaxial

Epitaxial, polycrystalline and amorphous dielectric layers for SD and IC technology

Epitaxial, heteroepitaxial semiconductor layers; polycrystalline semiconductor layers; dielectric layers for SD and IC technology

9

Gas sources

Si0 2 , Si 3 N 4 , polymers

Passivation layers in solid-state technology

Gas sources Si0 2 , Si 3 N 4 , Passivation layers polymers for SD and IC technology

Single-crys- Gas sources Epi-Si, talline, polyepi-G&As, crystalline, polyLSi, amorphous Si02, Si 3 N 4

Single-crystalline, polycrystalline, amorphous

6

[222, 3 4 0 373]

[183, 2 0 7 221, 289, 312-339]

[183, 2 0 4 206,293311]

[11-22, 4 4 54, 8 4 - 9 2 , 180-203, 238-288]

10

Laser-induced CVD

Electronbeam assisted CVD

Ion-beam assisted CVD

20

21

22

Any

Gases, volatile liquids

Any

Gases, vola- Any tile liquids

Gases, volatile liquids

0.1-2000

25 3

1 5 0 - 5 0 0 0.05

25-250

Amorphous

Amorphous

Amorphous, . pol ycry stalline, singlecrystalline

w, c

Si0 2 , Si 3 N 4

Si0 2 , Si 3 N 4 , metals

Gas sources Al, Au,

Gas sources

Gas sources

[223-231]

[232-235 290,374451]

Metal layers [236, 237] in solidstate technology

Passivation layers in solid-state technology

Passivation or metal layers in solid-state technology