CVD low-pressure

Dielectric Film Deposition. Dielectric films are found in all VLSI circuits to provide insulation between conducting layers, as diffusion and ion implantation (qv) masks, for diffusion from doped oxides, to cap doped films to prevent outdiffusion, and for passivating devices as a measure of protection against external contamination, moisture, and scratches. Properties that define the nature and function of dielectric films are the dielectric constant, the process temperature, and specific fabrication characteristics such as step coverage, gap-filling capabihties, density stress, contamination, thickness uniformity, deposition rate, and moisture resistance (2). Several processes are used to deposit dielectric films including atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), or plasma-enhanced CVD (PECVD) (see Plasma technology). 

Tsao,K.Y.,andBusta,H.H, Low Pressure CVD of Tungsten on Poly cry stalline and Single-Crystal Silicon viathe SiliconReduction, J. Electrochem. Soc., 131(ll) 2702-2708 (Nov. 1984)... 

The CVD reactions used to produce epitaxial silicon are described in Ch. 8. Originally, atmospheric-pressure CVD was used, but it is gradually being replaced by low-pressure CVD in spite of higher equipment cost and complexity. Low-pressure CVD appears to yield a better film with less autodoping and pattern shift. 

The most common IR window materials are zinc sulfide, which is translucent, and zinc selenide, which is transparent. Both of these materials are made by low-pressure CVD by the reaction of vaporized zinc and hydrogen sulfide or selenide (see Ch. 12). Germanium, another common IR window material, is also produced by CVD (see Ch. 8). 

Figure 5.2. Two of the more common types of low pressure CVD reactor, (a) Hot Filament Reactor - these utilise a continually pumped vacuum chamber, while process gases are metered in at carefully controlled rates (typically a total flow rate of a few hundred cubic centimetres per minute). Throttle valves maintain the pressure in the chamber at typically 20-30 torr, while a heater is used to bring the substrate up to a temperature of 700-900°C. The substrate to be coated - e.g. a piece of silicon or molybdenum - sits on the heater, a few millimetres beneath a tungsten filament, which is electrically heated to temperatures in excess of 2200 °C. (b) Microwave Plasma Reactor - in these systems, microwave power is coupled into the process gases via an antenna pointing into the chamber. The size of the chamber is altered by a sliding barrier to achieve maximum microwave power transfer, which results in a ball of hot, ionised gas (a plasma ball) sitting on top of the heated substrate, onto which the diamond film is deposited.

Low-Pressure CVD Processes. Low-pressure CVD (LPCVD) (—101 Pa) is the main tool for the production of polycrystalline Si dielectric and passivation films used in Si IC (integrated-circuit) manufacture (1, 20, 21). The main advantage of LPCVD is the large number of wafers that can be coated simultaneously without detrimental effects to film uniformity. This capability is a result of the large diffusion coefficient at low pressures, which... 

CVD reactors operate at sufficiently high pressures and large characteristic dimensions (e.g., wafer spacing) such that Kn (Knudsen number) << 1, and a continuum description is appropriate. Exceptions are the recent vacuum CVD processes for Si (22, 23) and compound semiconductors (156, 157, 169) that work in the transition to the free molecular flow regime, that is, Kn > 1. Figure 7 gives an example of SiH4 trajectories in nearly free molecular flow (Kn 10) in a very low pressure CVD system for silicon epitaxy that is similar to that described by Meyerson et al. (22, 23 Meyerson and Jensen, manuscript in preparation). Wall collisions dominate, and be-... 

Another issue in LPCVD reactor modeling is the transition to molecular flow for which the continuum formulation breaks down. This transition may be important in the modeling of very low pressure CVD Si epitaxy (22, 23). Monte Carlo simulations of free molecular flow in a very low pressure CVD reactor for Si epitaxy were illustrated in Figure 7 and discussed in an earlier... 

A brief review of the literature concerning the several materials employed in the fabrication of both TIR and ARROW structures is given in Table 2. The processes employed are completely different, ranging from molecular beam epitaxy to several chemical vapor deposition (CVD) systems, such as low-pressure CVD (LPCVD) or plasma-enhanced CVD (PECVD). As a rule, all suitable materials for ARROWS (and in general for IOCs) should have homogeneous refractive indexes, high mechanical and chemical stability, few... 

Polished or unpolished polysilicon (by low-pressure CVD at 620°C) is another etch mask option. Utilizing 2.5-pm-thick unpolished polysilicon, a maximum etch depth of 160 pm was reached using a HF/H20/HN03 (6 40 100) solution. Further etching causes large pits (1.5-2.2 mm dia.) to form on the glass. With polished poly silicon (1.5 pm thick), etch depth up to 250 pm can be achieved. When amorphous Si is used as the etch mask, a maximum etch depth of 170 pm can be reached [123]. 

Films grown by chemical vapor deposition are similar to the films described above, with one exception. The CVD process allows for the possibility of gas phase particle nucleation, and the incorporation of such particles in a growing surface can contribute to a surface roughness. This is one reason that atmospheric CVD is being used less and less, as compared to low-pressure CVD where such gas phase nucleation is less likely. 

The problem of assuring uniform depositions on many wafers closely spaced in a long uniform tube was solved when operation of the reactor at low pressure was considered.22 Normally, in an atmospheric pressure cold wall CVD system, the reactant gas is heavily diluted in N2 in order to reduce gas phase nucleation. At the pressures used for low pressure CVD (0.5-1.0 Torr), this is less of a problem so less diluent is needed. The net effect then is that deposition rates only fall by a factor of five. However, as many as 100 wafers can be processed in such a reactor at one time (see Figure 26), and this more than compensates for the lower deposition rate. In addition, due to the low pressure, diffusion occurs at high rates and the deposition tends to be controlled by the surface temperature. Given the very uniform temperatures available in a diffusion furnace, the deposition uniformity tends to be excellent in such a system. 

Rosier, R.S., Low pressure CVD production processes for poly, nitride,... 

In the mid-1970s, it was realized that low-pressure CVD processing could have significant advantages over atmospheric pressure systems. By reducing the pressure, it was found that the diffusion coefficient was sufficiently enhanced that deposition became surface controlled (see Chapter 1). In this case, wafers could be stacked closely and placed in a diffusion furnace to be processed... 

Apparently, the low-pressure CVD process leaves some chlorine in the layer, while the same process at high pressure does not. Since we do not understand the kinetic pathways for the formation of this film, we cannot predict how to modify it. However, by trial and error it was discovered that small additions of 02 to the N20 feed gas reduced the chlorine content to zero.4 Thickness uniformity was impacted by the 02 addition, but a value of 7% was obtainable. 

Up to this point, all of the films we have considered (Si02, Si3N4) were deposited under conditions such that they were amorphous. The only defects of interest were particles from the gas phase that might be incorporated into the growing film, or pinholes. Low-pressure CVD has reduced the incidence of particles, and thicker films can minimize the presence of pinholes. 

The lowest resistivity silicide film of the four we are considering is the TiSi2 film, so such films have always been of interest. A recent study14 has shown that these films can also be deposited by low-pressure CVD. For these experiments, a cold-wall reactor similar to the parallel-flow tube reactor sketched in Figure 17 of Chapter 1 was used. The wafer was heated by heating the susceptor from below by optical radiation. 

The most comprehensive experiments have been performed in low-pressure CVD hot tube reactors.24 When WF6 is reduced on clean, flat silicon surfaces, the deposition rate is very rapid (>1000 A/min) and self limiting. Generally, a tungsten film of less than 200 A (grown at 300° to 425°C and 500 mTorr) is sufficient to completely block this reaction, as shown in Figure 14. 

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