Silicon dioxide passivation

Silicon Dioxide (passivation layer) Acetic Acid 2 Ammonium Fluoride 1 Room... 

Fig. 6.7. Median life for aluminium corrosion, at 110°C/90% RH, as a function of the phosphorus content of silicon dioxide passivation.

The hazards associated with phosphorus-doped silicon dioxide passivations are now widely recognised and manufacturers have accordingly adopted control limits of 2-4% by weight of phosphorus. A few manufacturers are also making use of alternative forms of passivation layers, the three most popular of which are ... 

The relationship between delamination and failure for the lifetested specimens was more complex, and it was only for manufacturer F (the one that used a nitride passivation) that (i) delamination was a necessary precondition for failure, and (ii) the corrosion was restricted to the exposed aluminium at the bond-pads. For the other manufacturers, less than 50% of the lifetest failures delaminated prior to failure and, although the delaminated specimens showed signs of bond-pad corrosion, there were two other types of corrosion which were more prevalent. On the early failures there were small areas of severe corrosion scattered over the die and the frequency of such sites was correlated with that of passivation crack and pin-hole density for each manufacturer (up to 36 per die in the worst case), whereas for the late failures (and survivors) there was uniform corrosion of all the cathodi-cally biased tracks. This uniform corrosion was similar to that usually ascribed to high levels of phosphorus in CVD silicon dioxide passivations, so it would appear that this type of corrosion can even occur when, as in the cases reported here, the phosphorus contents are within the range 3-1-5-5%. 

Another example of a cold-wall reactor is shown in Fig. 5.9. It uses a hot plate and a conveyor belt for continuous operation at atmospheric pressure. Preheating and cooling zones reduce the possibility of thermal shock. The system is used extensively for high-volume production of silicon-dioxide coatings for semiconductor passivation and interlayer dielectrics. 

The uses of CVD silicon dioxide films are numerous and include insulation between conductive layers, diffusion masks, and ion-implantation masks for the diffusion of doped oxides, passivation against abrasion, scratches, and the penetration of impurities and moisture. Indeed, Si02 has been called the pivotal material of IC s.1 1 Several CVD reactions are presently used in the production of Si02 films, each having somewhat different characteristics. These reactions are described in Ch. 11. 

Silicon possesses an excellent oxide. Silicon dioxide (Si02) is a remarkably stable passivating layer, acts as a good electrical insulator, and forms an excellent interface... 

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. 

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. 

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... 

H. Flietner, Passivity and electronic properties of the silicon/silicon dioxide interface. Mater. Sci. Forum 185-188, 73, 1995. 

Semiconductor microchip processing often involves chemical vapor deposition (CVD) of thin layers. The material being deposited needs to have certain desirable properties. For instance, to overlay on aluminum or other bases, a phosphorus pentoxide-doped silicon dioxide coating is deposited as passivation (protective) coating, by the simultaneous reactions... 

In a non-hermetic package, the primary moisture barrier is typically a thin, (<0.1 micron) inorganic passivation layer of silicon dioxide or silicon nitride. However, passivation layers cannot withstand physical handling and they are not resistant to saline exposure. Additional protection is needed in the form of organic polymer coatings. 

The purpose of encapsulation is to protect electronic IC devices and prolong their reliability. Moisture, mobile ions, (eg., sodium, potassium, chloride, fluorides), UV-VIS and alpha particle radiation, and hostile environmental conditions are some of the possible causes of degradation or interaction which could negatively affect device performance or lifetime. Silicon dioxide, silicon nitride and silicon-oxy-nitride, commonly used as passivation layers have excellent moisture and mobile ion barrier properties and are, therefore, excellent encapsulants for devices. As for the... 

Passivate by thermal oxidation to form silicon dioxide on the surface. 

As with metals, semiconductors are also subject to passivation. Figure 22.9 shows the anodic dissolution and the passivation of n-type and p-type silicon electrodes in sodium hydroxide solution [13]. Silicon dissolves in basic solution in the form of soluble divalent silicon, Si(OH),iq or Si(OH)2jaq, and passivates forming a silicon dioxide film. 

The anodic passivation of semiconductors in aqueous solution occurs in much the same way as that of metals and produces a passive oxide film on the semiconductor electrodes. Figure 22.25 shows the anodic dissolution current and the thickness of the passive film as a function of electrode potential for p-type and n-type silicon electrodes in basic sodium hydroxide solution [32,33], As mentioned earlier, silicon dissolves in the active state as divalent silicon ions and in the passive state a film of quadravalent insoluble silicon dioxide is formed on the silicon electrode. The passive film is in the order of 0.2-1.0 nm thick with an electric field of 106 107 V cm 1 in the film within the potential range where water is stable. 

After etching the cavities, clear silicon dioxide membranes are obtained, and a short, about 20 s, overetching without passivation is applied in an Alcatel 601 DRIE machine to remove the polymer formed in the cavities during the DRIE process. 

A possible mechanism of formation of a sidewall passivation film in the case of silicon etching in a HCI/O2/BCI3 plasma is shown in Fig. 18 [77]. SiCf Hv-type byproducts are sputtered away by ion bombardment from the bottom of the trench. A portion of the sputtered flux strikes and sticks on the sidewalls on the trench. Oxygenation of the byproducts on the sidewalls results in a silicon dioxide type of film that resists etching. The sidewalls do not receive any appreciable ion bombardment and hence, depending on conditions, a rather thick inhibitor film may be formed. 

Wafer Bonding with intermediate Layers Wafers can be bonded using intermediate layers such as metals to form hermetic seals. Polymers, in general, do not provide water-tight sealing and are hence less important in the field of MEMS packaging. Soft metals such as indium can be bonded with silicon with a gold passivation [4]. Aluminum has been used to bond with silicon dioxide [5] and silicon nitride [6] to form hermetic seals. 

In the processing of integrated circuits, silicon dioxide (SiOa) can be used as a mask during ion implantation or diffusion of impurity into silicon, for passivation, for protection of the device surface, as interlayers for multilevel metallization, or as the active insulating material — the gate oxide film in metal-oxide-semiconductor (MOS) devices [1, 2], At the present time, several methods have been developed for the formation of Si02 layers, including thermal and chemical oxidation, anodization in electrolyte solutions, and various chemical vapor deposition (CVD) techniques [2, 3],... 

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