Silicon dioxide stress

Ceramic materials are typically noncrystalline inorganic oxides prepared by heat-treatment of a powder and have a network structure. They include many silicate minerals, such as quartz (silicon dioxide, which has the empirical formula SiO,), and high-temperature superconductors (Box 5.2). Ceramic materials have great strength and stability, because covalent bonds must be broken to cause any deformation in the crystal. As a result, ceramic materials under physical stress tend to shatter rather than bend. Section 14.22 contains further information on the properties of ceramic materials. 

The properties of silicon dioxide films also depend upon all plasma deposition parameters. Temperature is the critical parameter (240), although the compressive stress level varies with rf frequency (237, 240). Film topography can be varied during deposition by altering ion bombardment conditions (242, 243). In particular, the incorporation of Ar in the deposition atmosphere enhances sputtering and thus promotes conformal step coverage during film formation (243). 

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. 

One commercial wafer deflection gauge is available, and is sketched in Figure 6. The degree of light reflection is used to indicate the amount of wafer deflection. The only difficulty with this technique occurs when relatively low stress films are measured. For normal films (i.e., thermal CVD silicon dioxide) and a stress of 109 dynes/cm2, a typical 100-mm silicon wafer (0.62-mm thick) with a 1-jum thick film will deflect 10 jum at its center. The Ionic Systems gauge claims a 0.03-pm sensitivity, so the typical stress can be measured readily. For smaller stresses 108 dynes/cm2, it may be useful to use a thinned wafer to make deflection measurements. 

There are a number of subtle effects that have to be considered when making thin film stress measurements on silicon wafers First of all, the crystal orientation of the wafer Influences the resulting stress. The same thermal CVD silicon dioxide film thickness on the same substrate indicates larger tensile stresses on (100)-oriented wafers as compared with (111 (-oriented wafers. 

In addition to the construction of a multilevel interconnect network, the semiconductor industry also improves the performance of IC chips by incorporating low-resistivity metal wiring such as copper and new dielectric materials with lower k constant (see Section 1.3.1 for details). The added benefit of using low-fc dielectric materials includes a reduction in the crosstalk [29-31] and power dissipation [29-33]. The key challenge for the implementation of low-fc materials is related to their intrinsic weak mechanical properties. Furthermore, in order to achieve a k value below 2.2, practically all materials are made with pores that exacerbate mechanical stability issue [29-33]. This is a particular concern for the CMP community as the operation invariably involves mechanical stress and shear force. In addition, practically all low-fc dielectric materials are hydrophobic in nature. Lfpon exposure to moisture or wetness, the dielectric constant tends to increase. Therefore, unlike silicon-dioxide-based dielectric, the effective k constant may change after CMP. To... 

It must be stressed that the functions of a glidant and lubricant in a tablet formulation are totally different. A few materials, e.g. talc, can act as both glidant and lubricant, but usually two different excipients are needed. Thus, although colloidal silicon dioxide is an excellent glidant, it has no lubricant activity. Conversely, magnesium stearate, despite its popularity as a lubricant, can hinder rather than promote flow. 

In the following, a silicon nitride layer was deposited as the gate dielectric on a thermally oxidised silicon wafer. The nitride layer was re-oxidised to enhance the electrical stability. The silicon dioxide below the nitride film adopted the function of a buffer layer to reduce mechanical stress between the silicon and silicon nitride due to different thermal coefficients of expansion. To deposit the dielectric film, ammonia gas and triethylsilane were put into the process tube in a ratio of 1 5, at 800 °C and at a process pressure of 0.3 mbar. The thickness of the deposited dielectric film was about 75 nm in total. 

The fumed silicon dioxides are perhaps the most effective glidants. These are materials with very small (10 nm) spherical particles that may achieve their glidant properties by rolling over each other under shear stress. They are available in a number of grades with a range of hydrophobic and hydrophillic forms. 

Protection layer of the photoresist is essential in this technological chain. If this layer is not used, the membranes of silicon dioxide would crack due to internal stress after being released from the substrate. Cracking the membranes results in a breaking on the cantilevers and brings technological yield close to zero. 

Blech, 1. A. and Cohen, U. (1982), Effects of humidity on stress in thin silicon dioxide films, Journal of Applied Physics 53, 4202-4207. 

Undoped CVD silicon dioxide which is thin enough (2000 A) to withstand the built-in stress without cracking. 

Silicon nitride has received the most attention, but only a few manufacturers are using it commercially because it is difficult to control some of the properties which can affect the operation of an IC (such as hydrogen content and stress). The main attribute of silicon nitride is that it is impervious to moisture and this makes it the ideal passivation when highly phosphorus-doped silicon dioxide layers are used to separate two levels of interconnection, as, for example, in gate arrays. (One requirement of an inter-layer dielectric is that it should smooth out underlying steps, and 6-10% phosphorus-doped silicon dioxide has the appropriate flow properties to do this.) In such a case silicon nitride prevents the uniform cathodic corrosion which is associated with the use of such oxide layers as passivations. 

P Zorabedian, F Adar. Measurement of local stress in laser-recrystallized lateral epitaxial silicon films over silicon dioxide using Raman scattering. Appl Phys Lett 43 177-179, 1983. 

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