In regard to the CVD of insulating films in general, and silica films in particular, three general reactors are presently used: atmospheric pressure CVD (APCVD), low and medium temperature low pressure CVD (LPCVD), and plasma-enhanced CVD (PECVD). LPCVD is often further divided into low and high temperatures.

APCVD systems allow for high throughput and even continuous operation, while LPCVD provides for superior conformal step coverage and better film homogeneity. PECVD has been traditionally used where low temperatures are required, however, film quality is often poor. As compared to PECVD, photo-assisted CVD has the additional advantage of highly selective deposition, although it has been little used in commercial systems. Table 1 summarizes the advantages and disadvantages of each type of CVD system commercially used for SiO₂ films.

The requirements of CVD films for electronic device applications have become increasingly more stringent as device sizes are continually reduced. Film thickness must be uniform across an entire wafer, i.e., better than ±1%. The structure of the film and its composition must be controlled and reproducible, both on a single wafer, as well as between wafer samples. It is also desirable that the process is safe, inexpensive, and easily automated.

A number of variables determine the quality and rate of film growth for any material. In general, the deposition rate increases with increased temperature and follows the Arrhenius equation, Equation 1, where R is the deposition rate, E_a is the activation energy, T is the temperature (K), A is the frequency factor, and k is Boltzmann's constant (1.381 x 10^-23 J/K).

(1)

At the high temperatures the rate of deposition becomes mass transport limited. Meaning, the rate of surface reaction is faster than the rate at which precursors are transported to the surface. In multiple source systems, the film growth rate is dependent on the vapor phase concentration (or partial pressure) of each of the reactants, but in certain cases the ratio of reactants is also important, e.g., the SiH₄/O₂ growth of SiO₂. Surface catalyzed reactions can also alter the deposition rate. Such as the non-linear dependence of the deposition rate of SiO₂ on the partial pressure of Si(OEt)₄. Gas depletion may also be significant requiring either a thermal ramp in the chamber and/or special reactor designs. The necessary incorporation of dopants usually lowers deposition rates, due to competitive surface binding.

For the applications of insulating materials as isolation layers, an important consideration is step coverage: whether a coating is uniform with respect to the surface. Figure 1a shows a schematic of a completely uniform or conformal step coverage of a trench (such as occurs between isolated devices) where the film thickness along the walls is the same as the film thickness at the bottom of the step. Uniform step coverage results when reactants or reactive intermediates are able to migrate rapidly along the surface before reacting. When the reactants adsorb and react without significant surface migration, deposition is dependent on the mean free path of the gas. Figure 1b shows an example of minimal surface migration and a short mean free path. For SiO₂ film growth LPCVD has highly uniform coverage (Figure 1a) and PECVD poor step coverage (Figure 1b).

Figure 1: Step coverage of deposited films with (a) uniform coverage resulting from rapid surface migration and (b) nonconformal step coverage due to no surface migration.

The general requirements for any CVD precursor have been adequately reviewed elsewhere, and will not be covered here. However, many of the gases and organometallics used to deposit dielectric films are hazardous. The safety problems are more severe for LPCVD because the process often uses no diluent gas such as argon or nitrogen. Table 2 lists the boiling point and hazards of common inorganic and organometallic precursor sources for CVD of SiO₂ and doped silica. Many of the precursors react with air to form solid products, thus leaks can cause particles to form in the chamber and gas lines.

In principle, the deposition of a SiO₂, or silica, thin film by CVD requires two chemical sources: the element (or elements) in question, and an oxygen source. While dioxygen (O₂) is suitable for many applications, its reactions may be too fast or too slow for optimum film growth, requiring that alternative oxygen sources be used, e.g., nitrous oxide (N₂O) and ozone (O₃). A common non-oxidizing oxygen source is water. A more advantageous approach is to incorporate oxygen into the ligand environment of the precursor, and endeavor to preserve such an interaction intact from the source molecule into the ultimate film; such a source is often termed a "single-source" precursor.

The most widely used method for SiO₂ thin film CVD is the oxidation of silane (SiH₄), first developed in 1967 for APCVD. Nonetheless, LPCVD systems have since become increasingly employed, and exceptionally high growth rates (30,000 Å/min) have been obtained by the use of rapid thermal CVD.

The chemical reaction for SiO₂ deposition from SiH₄ is:

(2)

At high oxygen partial pressures an alternative reaction occurs, resulting in the formation of water.

(3)

While these reactions appears simple, the detailed mechanism involves a complex branching-chain sequence of reactions. The apparent activation energy is low (< 41 kJ/mol) as a consequence of its heterogeneous nature, and involves both surface adsorption and surface catalysis.

Nitrous oxide (N₂O) can be used as an alternative oxygen source to O₂, according to the overall reaction, Equation 4.

(4)

A simple kinetic scheme has been developed to explain many of the observed aspects of SiH₄-N₂O growth. It was suggested that the reaction is initiated by decomposition of N₂O, Equation 5, generating an oxygen radical which can abstract hydrogen from silane forming a hydroxyl radical, Equation 6, that can react further with silane, Equation 7.

(5)

(6)

(7)

Evidence for the reaction of the OH radical to form water is the formation of a small quantity of water observed during the oxidation of SiH₄. Silyl radicals are oxidized by N₂O to form siloxy radicals, Equation 8, which provide a suitable propagation step, Equation 9.

(8)

(9)

It has been proposed that the silanol (SiH₃OH) is the penultimate film precursor.

The SiH₄-O₂ and SiH₄-N₂O routes to SiO₂ thin films are perhaps the most widely studied photochemical CVD system of all dielectrics. Photo-CVD of SiO₂ provides a suitable route to deposition at low substrate temperatures, thereby avoiding potential thermal effects of wafer warpage and deleterious dopant redistribution. In addition, unlike other low temperature methods such as APCVD and PECVD, photo-CVD often provides good purity of films.

A summary of common silane CVD systems is given in Table 4.

The most widely used process of the high temperature growth of SiO₂ by LPCVD involves the N₂O oxidation of dichlorosilane, SiCl₂H₂, Equation 10.

(10)

Deposition at 900 - 915 °C allows for growth of SiO₂ films at ca. 120 Å/min; however, these films are contaminated with Cl. Addition of small amounts of O₂ is necessary to remove the chlorine.

While PECVD has been employed utility halide precursors, the ability of small quantities of fluorine to improve the electrical properties of SiO₂ has prompted investigation of the use of SiF₄ as a suitable source.

The first CVD process to be introduced into semiconductor technology in 1961 was that involving the pyrolysis of tetraethoxysilane, Si(OEt)₄ (commonly called TEOS from tetraethylorthosilicate). Deposition occurs at an optimum temperature around 750 °C. However, under LPCVD conditions, the growth temperature can be significantly lowered (> 600 °C). The high temperature growth of SiO₂ from TEOS involves no external oxygen source. Dissociative adsorption studies indicate that decomposition of the TEOS-derived surface bound di- and tri-ethoxysiloxanes is the direct source of the ethylene.

PECVD significantly lowers deposition temperatures using TEOS, but requires the addition of O₂ to remove carbon contamination, via the formation of gaseous CO and CO₂, which are subsequently not incorporated within the film. Although deposition as low as 100 °C may be obtained, the film resistivity increases by three orders of magnitude by depositing at 200 °C; being 10¹⁶ Ω.cm, with a breakdown strength of 7 x 10⁶ V/cm.

Addition of O₂ for APCVD growth does not decrease the deposition temperature, however, if ozone (O₃) is used as the oxidation source, deposition temperatures as low as 300 °C may be obtained for uniform crack-free films. It has been postulated that the ozone traps the TEOS molecule on the surface as it reacts with the ethoxy substituent, providing a lower energy pathway (TEOS-O₃ @ 55 kJ/mol versus TEOS-O₂ @ 230 kJ/mol and TEOS only @ 190 kJ/mol).

There are significant advantages of the TEOS/O₃ system, for example the superior step coverage it provides. Furthermore, films have low stress and low particle contamination. On this basis the TEOS/O₃ system has become widely used for silica, as well as silicate glasses.

A wide range of alternative silicon sources has been investigated, especially with regard to either lower temperature deposition and/or precursors with greater ambient stability.

Diethylsilane (Et₂SiH₂), 1,4-dislabutane (DBS, H₃SiCH₂CH₂SiH₃), 2,4,6,8-tetramethylcyclotetrasiloxane (TMCTS, Figure 2a, where R = CH₃), and 2,4,6,8-tetraethylcyclotetrasiloxane (TECTS, Figure 2a, where R = C₂H₅), have been used in conjunction with O₂ over deposition temperatures of 100 - 600 °C, depending on the precursor. Diacetoxydi-tert-butyl silane (DADBS, Figure 2b) has been used without additional oxidation sources. High quality silicon oxide has been grown at 300 °C by APCVD using the amido precursor, Si(NMe₂)₄ (Figure 2c).

Figure 2: Alternative organometallic silicon sources that have been investigated for the growth of silica thin films.

An interesting concept has been to preform the -Si-O-Si- framework in the precursor. In this regard, the novel precursor T₈-hydridospherosiloxane (H₈Si₈O₁₂, Figure 2d) gives smooth amorphous stoichiometric SiO₂ at 450 - 525 °C by LPCVD. The decomposition mechanism in the presence of added oxygen involves the loss of water, Equation 11. IR studies indicate that the Si-O-Si bonds are preserved during deposition. While films are of high quality, the present synthesis of H₈Si₈O₁₂ is of low yield (ca. 21%), making it currently impractical for large scale processing.

(11)

Films of BSG, PSG, and BPSG may all be grown from SiH₄, O₂ and B₂H₆ and/or PH₃, at 300 - 650 °C. For APCVD, the reactants are diluted with an inert gas such as nitrogen, and the O₂/hydride molar ratio is carefully controlled to maximize growth rate and dopant concentration (values of 1 to 100 are used depending on the application). Ordinarily, the dopant concentration for both BSG and PSG decreases with increased temperature. However, some reports indicate an increase in boron content with increased temperature. Film growth of BPSG was found to occur in two temperature regions. Deposition at low temperature (270 - 360 °C) occurred via a surface reaction rate limiting growth (E_a = 39 kcal/mol), while at higher temperature (350 - 450 °C), a mass-transport rate limited reaction region is observed (E_a = 7.6 kcal/mol).

LPCVD of BSG and PSG is conducted at 450 - 550 °C with an O₂:hydride ratio of 1:1.5. Conversely, an O₂:hydride ratio of 1.5:1 provides the optimum growth conditions for BPSG over the same temperature range. The phosphorous in PSG films was found to exist as a mixture of P₂O₅ and P₂O₃, however, the latter can be minimized under the correct deposition conditions. Some difficulties have been reported for the use of B₂H₆ due to its thermal instability. Substitution of B₂H₆ with BCl₃ obviates this problem, although the resulting films are invariably contaminated with 1 weight per cent chloride.

Arsenosilicate glass (AsSG) thin films are generally grown by APCVD using arsine (AsH3); the use of which is being limited due to its high toxicity. However, arsine inhibits the gas phase reactions between SiH₄ and O₂, such that film grown from SiH₄/AsH₃/O₂ show improved step coverage at high deposition rates.

As with SiO₂ deposition, see above, there has been a trend towards the replacement of SiH₄ with TEOS on account of its ability to produce highly conformal coatings. This is particularly attractive with respect to trench filling. Furthermore, films of doped SiO₂ glasses have been obtained using both APCVD and LPCVD (typically below 3 Torr), with a wide variety of dopant elements including: boron, phosphorous, and arsenic, including antimony, tin, and zinc.

Boron-containing glasses are generally grown using either trimethylborate, B(OMe)₃, or triethylborate, B(OEt)₃, although the multi-element source, tris(trimethylsilyl)borate, B(OSiMe₃)₃, has been employed for both silicon and boron in BPSG thin film growth. Similarly, whereas PH₃ may be used as the phosphorous source, trimethylphosphite, P(OMe)₃, and trimethylphosphate, O=P(OMe)₃, are preferred. Likewise, triethoxyarsine, As(OEt)₃, and triethylarsenate, O=As(OEt)₃, have been employed for AsSG growth.

The co-reaction of TEOS with organoboron and organophosphorous compounds allows for deposition at lower temperatures (500 - 650 °C) than for hydride growth of comparable rates. However, LPCVD, using an all organometallic approach, requires P(OMe)₃ because the low reactivity of O=P(OMe)₃ prevents significant phosphorus incorporation. Although premature decomposition of P(OMe)₃ occurs at 600 °C (leading to non-uniform growth), deposition at 550 °C results in high film uniformity at reasonable deposition rates.