Final Passivation

Dopant species can be codeposited with the Si02 by introducing small amounts of the dopants in hydride or haUde form. P-doped Si02, called P-glass, functions as an insulator between polysiUcon gates and the top metallisation layer of ICs. It is also used as a final passivation layer over devices, and as agettering source (17). 

Finally, passive samplers have also been developed for ozone, primarily for use in epidemiological studies. For example, Brauer and Brook (1995) describe the application of a passive sampler in which air containing ozone diffuses through a Teflon membrane and reacts with nitrite. The sampler is then extracted and the nitrate product measured using ion chromatography. 

Silicon dioxide films have been an essential factor in the manufacture of integrated circuits from the earliest days of the industry. They have been used as a final passivation film to protect against scratches and to getter mobile ion impurities (when doped with phosphorus). Another application has been as an interlayer dielectric between the gate polysilicon and the aluminum metal-ization. Initially, most such films were deposited in atmospheric pressure systems. In recent years, low pressure processes have assumed greater importance. We will begin by examining the atmospheric process. 

The low-temperature depositions described in the present section can be used for either interlayer dielectrics or final passivation films. Their primary disadvantage is one of film quality, because the process is susceptible to gas-phase nucleation and incorporation of particles into the film. 

Attention was then turned to a higher temperature, low-pressure process. Although such films would not be useful for final passivation, they could provide valuable interlayer dielectrics. At first. SiH4 with N20 was tried, where the reaction would proceed as follows ... 

When phosphorus is added to Si02, in addition to gettering mobile alkali ions, it tends to reduce the intrinsic tensile stress in such films, thereby reducing their tendency to crack. Both functions are important when the film is used as a final passivation film for integrated circuits encapsulated in plastic. Phosphorus additions of 7 weight percent seem to be optimum in order to produce the above desirable film characteristics. 

PECVD of silicon nitride has been of commercial importance since 1976.1 The original motivation was to find a final passivation layer for an integrated circuit that would replace the doped silicon dioxide films then in use. The latter were not reliable enough to permit packaging of integrated circuits in plastic. Silicon nitride was recognized as a better final passivation film, but the only available technique for its deposition was the high-temperature thermal process. Since it had to cover an aluminum final metallization layer that would melt at 600°C, this clearly could not work. The solution was to use PECVD at 350° to 400°C. 

Earlier, we reviewed silicon dioxide (thermal) films deposited with added phosphorus to serve as a getter for mobile ion impurities, as a final passivation film. Plasma-enhanced silicon nitride can also be doped with phosphorus.6 Some of the film characteristics have been reviewed, and it was found that the films with 2 to 3% P had the best electrical quality. No measurements of stress or H2 content were reported, so it is not clear that these would be use-able films. 

Since one major motivation behind the use of plasma BPSG was to provide an improved passivation barrier, the better crack resistance is an advantage, but the greater sodium penetration is a negative. Therefore, it is not clear if it would be advantageous to make this replacement11 for a final passivation film. Its use as an inter-metallic dielectric may be more useful. 

Finally, we will consider PECVD silicon oxynitrides, and their unique characteristics. When oxygen is added to a PECVD nitride film, there are indications that it may improve its crack resistance as a final passivation layer.13 Also, there may be advantages in terms of its electrical characteristics as an interlayer dielectric. Therefore, the nature of films grown when N20 is added to a SiH4, NH3 and He gas mixture in a high frequency (13.56 MHz), cold-wall, parallel-plate reactor have been studied. 

Although the surface of most IC chips has been passivated with a layer of inorganic dielectric material such as silicon dioxide or silicon nitride (polyimides have also been used as final passivating layers), the protection provided by such layers is not sufficient to ensure reliable operation throughout the lifetime of the device. The three basic methods of protection are... 

Crystal surface modification The surface of the crystals may also be modified by the final passivation rinse. When a final rinse containing chromate or Cr(III) is used, a thin film of either ZnCr04, CrP04 [32], or CrOOH [33] is formed on the surface. Likewise, when steel is used as the substrate material, and in the presence of passivating additives such as nitrite, a thin layer of FeP04 may be formed on the surface of the crystals. Further modification of the phosphate surface may be induced if the crystals come into contact with an alkaline environment, as described below. 

The final passive mechanism is nonionic diffusion. In this case, the solute diffuses as the undissociated acid through the cell membranes, which act as lipid barriers. Concentration difference of the undissociated acid is the driving force. The extent of this process is determined by three factors. The first two, pH of the medium and pKa of the bile acid, define the proportion of molecules in the undissociated state. The third factor, solubility in the lipid membrane, determines the ease of penetration. Taurocholic acid, by virtue of its poor solubility in lipid-like phases and its very low pKa, does not participate significantly in this process. On the other hand, unconjugated cholic acid and other unconjugated acids which have appropriate physiochemical properties can be extensively absorbed by this mechanism. Under normal... 

---